Seconding the biphobia theory and then some. Lesbians tend to look at trans men as "safe" territory, thinking that someone born with female parts/living a female life will be more sensitive to their issues while adding the benefits of looking straight to social circles etc. A lot of these women are just ignorant about trans people and lack the understanding that everyone has a unique experience/view on being trans etc, hence singling out trans men while rebuffing trans women. Parts(or assumed parts) and how they work play a big role in this, as well. There's just this big assumption that all trans men have vaginas and breasts and that they're just playing dress up and vice versa for trans women. If you're attracted to certain parts, and you assume that someone you might find attractive has them, you might pursue them regardless of what they say or feel. It's...an issue I find to be complicated and hard to describe.
That being said, I do find it offensive when lesbians single out trans men as a special gender and chase them for dating, especially while rebuffing trans women and bashing guys who have medically transitioned etc. Thankfully, I've not had this happen to me, but I've seen other people get "chased" and it is unnerving.
What I have to disagree with, though, is disputing one's sexuality based on what they find pleasurable in the bedroom and stating that straight men/women cannot have successful relationships with trans people and still be straight. I know plenty of straight dudes that like pegging/having their girlfriends top them in the bedroom sometimes. And there are lesbians who really enjoy penetration etc. The sexual act itself does not determine the person you're attracted to, kiddo. If you're not keen on ~the buttsex~ or any other form of penetration, don't do it. You know what you like, and that's what's important. Just don't pin your preferences on other people and try to relabel their sexuality. Just don't.
And then there are also many successful relationships between cis and trans people. Trans men are men, and trans women are women. No dispute there. I tend to view it like this: even cis people have varied bodies. If a straight chick gets to know a trans dude and he comes out to her when she expresses interest and it doesn't put her off, it doesn't make her bisexual or pansexual. Only she can determine that for herself. She's gotten to know this guy, she likes him. Hell, maybe she loves him and views him as a normal human being who isn't just his parts. Is there really anything wrong with that?