Penetrative sex before bottom surgery...depends on the guy. His size, how hard he is at the moment, etc.
But an erection in general is basically just blood rushing to ones penis to enlarge it in preparedness for sex. If that's what you mean, of course transmen have erections, just like anybody else. Anybody with testosterone flowing through his body will tend to get a hard-on every once in awhile at least, if not often. A transman's dick, just like any other dick, is made of erectile tissue. Of course, erections are a little different with T-guys usually, as it's less "stand up" and more "stick out," but everyone's erections are different.
Anyway, if what you're asking is "Is there really a difference between a transman's flaccid and erect penis where it would warrant calling it an 'erection'?" then my personal answer, in my experience, is yes. There is definitely a difference in size/hardness/etc between my penis when it's flaccid and my penis when it is erect.
Check out X-tube; there are some fairly well-endowed transguys on there where their erections are pretty noticeable.
