The main problems are as follows:

1. Expecting trans women to let you live out your phallocentric misogynistic sex fantasies about "woman, but peepee?"
2. Expecting trans women to let you live vicariously through them because you're obviously repressing your own trans feelings
3. Expecting trans women to be "easy" because their potential dating pool is tiny and they're in prime position to be abused

It's obviously good to be attracted to trans people, but these are the types of behaviors that typify most actual chasers: very objectifying and harmful behavior. Even at its most innocuous, primarily dating trans women due to them being trans is obviously othering trans women from being "full" women.

In addition to everything @Erika3sis@hexbear.net already said, chasers have a tendency to be massive creeps and sex pests with giant entitlement issues. Sexual harassment from chasers whith heavy transphobic undertones is something a lot of trans people have to deal with, and they regularly have the audacity to make demands about our genitals, trying to talk trans women out of bottom surgery and things like that. They completely objectify us and generally treat the people they're dating like shit.

A Chaser isn't just "somebody attracted to trans people", a lot of people are attracted to us without being chasers. I mostly date other trans people myself, i have a good friend and comrade who's a cis dude and had a relationship with a trans woman and i'd never call him a chaser, but there are certain types we have to put up with, and they're extremely common and share certain extremely eeeew and problematic and sometimes outright criminal patterns of behavior that are brought up in trans communities over and over again. Defining a chaser varies from person to person because we've all made our own negative experiences, but there are very clear patterns that show how mysogyny and transphobia intersect in a lot of men (and some cis lesbians) that lust after us.

This isn't made better by the fact that dating trans people comes with certain strings attached. For example, if somebody would only be attracted to how i look right now, any relationship beyond me giving them a pity fuck would be unworkable because i will not look like this a year from now, i will have entirely different body parts two years from now and these changes are entirely non-negotiable, in fact my ability to live how i want to hinges on these changes happening. When i date other trans women, i always make sure to let them know that i desire them they way they are and the way they will be, that i am here to keep them company on their journey and to give acceptance both to their present self and to how they are becoming. To ease their dysphoria today and help them through the rough patches tomorrow. Because we all need that. Particularly because it's a ton of pain and stress and work to go through bottom surgery and recover from it. I do not need a guy who likes girls with dicks, i need somebody who will carry my groceries when my crotch is still a swollen, healing wound and who will offer a shoulder to cry on when dilating gets to me. Trans bodies do not stay the same, our transformations include both unspeakable joy and massive struggle for us and anybody who narrows which trans bodies they can be attracted to down to a set of criteria that are directly at odds with transition timelines should just hands down never date a trans person, it cannot work. And no trans person out there needs to hear about such hangups, we do not need to have that conversation with anybody, yet chasers continue to think it's a good pickup line to ask about our genitals.

I'm not saying cis people are not fit to date trans people, some are, but all of you need tons of education and explaining until a relationship with you can work comparably well as one between two trans people tends to work naturally, and that's assuming you're genuinely in love with the trans person in question. That's just how it is because our lives and all the shit we have to put up with are so vastly, incredibly different from mainstream experiences. Somebody who's seen some completely unrealistic porn depiction of how a trans woman on viagra is economically coerced to fuck like a man and now wants to have that transphobic fantasy irl will never be able to give us anything we need, they can only be a threat to our wellbeing.