It's pretty much the fact that suddenly a penis springs off of a hot chick. Trap sprung. It's an unusual thing (most women don't have penises; a lot of young men haven't seen pictures of women with penises, and most people do not see a woman with a penis irl) that causes shock, surprise, and strange questions and reactions (including a lot of "I'd still hit it" from certain guys on any given forum.)
I mean let's face it, if you actually simulate that in real life... get in bed with a guy, he takes your panties off, and out pops a johnson, you did trick him. All of his expectations and desire just suddenly get blasted sideways. Reality is that we're different than regular women and that we must accommodate that (as must our partners.) Post op, obviously, many women decide not to disclose to casual partners... and sometimes not to serious partners, either. All of that is a very murky bog.
Frankly, if you're in your teens/early 20s, "Yeah, uhm, I'm a trap" might be an icebreaker for the trans conversation with an internet-aware guy or girl. The more comfortable and sure you are of presenting yourself to someone, the more confidence you will inspire.
Today a male coworker of mine (father of one, adopted two sisters from a foreign country) asked if my boobs were real now because he remembered I mentioned padding (he misunderstood my pushup bra for stuffing my bra.) Later in the conversation he said that he brought it up because of one of his daughter's breasts growing in and the discomfort she causes him with either a) showing off the nipples through her clothing or b) wearing a padded bra that enhances her size considerably for her age.
Of course, since he asked about my boobs, I just started explaining the difference between pushup bras and regular bras, and how most bras have some degree of lining as a comfortably full shape makes clothes fit better, is more aesthetically pleasing, but also, that that layer of padding protects the breasts from impact injury and elaborated on how often I hurt my boobs back before I got to wear a bra at work.
And explained that my boobs are up to a B, but I wear a C because I have a broad chest and need the wider cup to accommodate the width of my boobs, and that I need a new pushup bra so I fill the cup (which I don't with my non pushup C.) I didn't get the chance to talk about how I'm too big for my B sports bra, and how underwire can actually be more comfortable if you can't get a soft bra to hold them in place.
He took it all in stride. I was trying to make him uncomfortable, but he rolled with it - he's got 3 daughters after all. And maybe I educated him about boobs, 'cause they certainly won't :p
A lot of people here would probably be really offended... perhaps I'm just used to fielding a lot of questions from my coworkers / extended family as they sort of feel like, but ->-bleeped-<-, I'd rather flat out talk about something like that rather than have someone going "pssspspspsp think her's are real? do they really grow boobs?"
I'm never doing this ->-bleeped-<- again at another job, but then I won't have to... I'll have enough stealth to avoid the issue. But I've been here too long to hide it. I'd rather just throw it out there and deal with it than try to downplay it in this scenario. There's also been a surprising amount of people, who when I am outed to, relate how they knew someone who transitioned. Sometimes more than one person. The more that the average person can relate, at least in passing, to people in our situation, the easier it will get for us.