Since the similar thread I started was going nowhere fast, I'll just join this one.

Transmen are more likely to remember your size, because it's not a totally alien system to him. (Depending, of course, on how old he was when he started buying from the men's department.) Though I'll still say that Men's sizes make so much more sense.
Transguys can get it up and keep it up any time without Viagra.
You'll never hear a Transguy say, "Oh come on, honey. You don't want to give me blue balls do you?"
Transguys will never be heard saying, "Oh, what's you're problem? You on the rag or something?"
Someone asked what's in it for gay guys? How about ...
If you go to a formal dance, transguys have already been taught how to follow. (Though that's always been a huge problem for me. I always want to lead.

)
As mentioned before - you can comfortably accommodate anything from a virgin to a size queen.
Transmen usually look young for their age - a plus in the very youth-centric gay community.