This is a topic with a lot of nuance, so please read carefully.
To begin, my background:
I have been a rights activist as long as I can remember, I have 5 years post-grad education but in varied fields (meaning I'm a PhD in number of classes, but more a jill-of-all-trades instead of a specialist), I was raised by two psychologists (who definately raised me to understand their field, so I have a lot of the technical knowledge but none of the bedside manner), and who went into biotech and the sciences, and I'm also a trans woman. So, this is answer is going to be probably one of the most correct answers you're going to get, but also one of the most technical, but I'll try to keep it as understandable as possible, but it's going to be long-winded, so... I'm sorry for that.
So, now to your answer.
First, before I go further, I want to say that I'm not offended currently. And I'm not telling you who you should date or shouldn't, nor am I telling you that it's transphobic not to fall in love with certain people. What I am trying to do is tell you how to say what you're apparently trying to say, instead of what you said.
Secondly, just so you know, the question you just asked, the way you asked initially, is borderline transphobic, and is considered one of the classic "accidentally transphobic" questions. However, you're also asking the question of how to ask, so definitely putting this in the 'accidental' category.
Now, before you get angry about me calling your question transphobic, I'm going to explain why. This is not an angry explanation, or a "Well, why are you rejecting so-and-so and you should love them even if you don't" explanation, I am simply pointing out why this question is a social minefield, and am encouraging you to re-think how you approach the question.First off, who you are attracted to and not attracted to is valid. However, you're very likely putting a category of attraction that is attributed to trans women that isn't a valid way of categorizing trans women.
I'll start off with the basic, and then go into detail.
The problem with assuming you aren't interested in trans women is the fact that by assuming you're not attracted to trans women is you're making an assumption that you can tell the difference between a cis woman and a trans woman. This is simply not the case. You may be able to tell the difference between a lot of cis women and a lot of trans women, but you cannot do this absolutely.
If you claim you are "not interested in trans women", this implies there is a feature of trans women you do not find attractive. The problem here, is that there are no 100% shared features among trans women. Anything that you find a "turn off about trans women" doesn't apply to all trans women.
For example, when you say "I'm not interested in trans women", I'm going to imagine for a moment that you mean, "I'm not interested in someone who has a masculine build, and parts seen by non-progressive society as masculine, but who presents female". This is how popular media frequently portrays trans women. Less than a tenth of trans women fall in this category. This is what's known as "2nd puberty", and depending on the trans woman, it only lasts 2 - 5 years. A better way to say this is, "I'm not interested in clockable trans women"If, when you say, "I'm not interested in trans women", we imagine you mean, "I'm not interested in someone who has a d--k", many, many trans women are post-op, and have female genitals pretty much the same as any other girls. A better way to say this is, "I'm not interested in pre-op trans women."
If, when you say, "I'm not interested in trans women", we imagine you mean, "I'm not interested in someone without a uterus", not only does that leave out most trans women (uterus transplants are now a thing as of last year, so won't leave out all trans women for long), but it also leaves out a lot of cis women. At which point, you'd say, "I'm not interested in women that don't have a uterus."
Maybe, if when you say, "I'm not interested in trans women", you want someone who was raised all their lives in female culture. (For the moment, avoiding the complexity that female culture changes from region to region), some lucky trans women were identified early, and transition very early in life in an accepting culture, and never go through a masculine puberty, attend school as a girl, dress as a girl, have been raised purely feminine and masculinity is just as completely an alien topic to them as it is to you. Similarly, especially in Islamic countries, there are cis women who are raised 100% male (usually to families without a son, meaning this is a woman you probably wouldn't care for either). At which point, you'd say, "I'm not interested in women who were raised in male culture."
A point to keep in mind. Trans women are women. And I don't mean this in some social "you are who you choose to be" sense. There is a biological inside-the-body reason most trans women "choose" to be women, and it's the fact that they are women1. So, if you are excluding trans women, you are excluding women. So if you have preferences in the women you date, state those preferences, don't just lump a bunch of unrelated women into one category.
Instead of blanket-banning trans women as a cohesive group (which they aren't a cohesive group, anymore than 'women who have six toes'), focus on the particular traits that you associate with trans women that you consider a turn off. Be aware that whatever those traits are, not all trans women have those traits, and some cis women likely do have those traits. There is no magical dividing line between trans women and cis women.
1 Many trans women would best be described as women with a birth defect. As the body develops in the womb, it frequently checks its environment to see, "Should I be building masculine parts or feminine parts?" One of the most common things checked for during this process is testosterone. If the mother gets enough testosterone in their system (for example, this can be caused by stress), it can trigger a change in the baby's development. One of the earliest things developed is the nervous system and among the last things developed is skeletal, muscular, and reproductive systems. So frequently a trans woman is a girl who was developing in the womb who her mom's body skipped a horomonal beat, and decided to finish up by tacking on masculine bits. In other words, these trans women are literally women who were trapped in a man's body. They were, biologically, a woman who got a d--k slapped on them at the end before they entered the world. [Note: This is only one among many ways a trans woman can become a trans woman, and is used demonstratively, there are other ways, which include chimerism, epigenetic factors, genetic anomolies, and more. I can get into more detail if asked.)
Edit: This isn't saying that women have to go through some biological test to be valid. They
used to do that. That
used to be a requirement. It cost hundreds of thousands of dollars. Insurance companies did research, and found that asking people their choices and identity (along with a counselor to filter out some mental illnesses that caused people to be confused), turned out it was nearly a 1:1 perfect substitute. This is why we call it "choice" now. It's not a choice, but because letting people choose ends up with the right answer. Kind of like if you asked someone, "Is your arm cut off, are you in pain?" and they answer "Yes", good chance their arm is cut off and they're in pain. There
used to be a pattern of telling people they had to go through certain tests to valid, and then it only caught those trans women that the particular test was able to catch (when there's tons of types). Those who used to insist on 'medically validated' transness didn't do any good, but only gatekept against lots of valid trans women. They were called truscrum, don't do that.
