I think the author clarified at the end that it was crossdressing.
There are kinks like that in which men will crossdress and do roleplay in which they're being dominated and/or treated like a woman but it's not necessarily tied to being trans, just feminisation kink.
As for the seme's boobas idk ⊙.☉, but author said "playing with nipples too much" or "second sex characteristic" which makes me think that they're just overstimulated or naturally big bc some men develop more estrogen.
At the end the author gives us their profiles, in which they use he/him pronouns. So that's why I deducted they liked roleplay and crossdressing, kouhai likes being pretty and senpai finds comfort in the praise he receives when they live stream.
is the seme supposed to be trans woman who hasn’t fully transitioned, yet? because it would make sense why they have breasts and “crossdresses.” also the way they talk about female genitalia in a man’s body (man pussy and whatnot), it seems that they want the uke to feel like a woman which can be them projecting their gender dysphoria or something? or maybe im reading too much into it. idk what tf i just read but if they’re supposed to be a trans woman, this should not be listed as yaoi.
EDIT:
my assumptions are very ignorant and uneducated, i apologize. there is a reply under this comment that explains it thoroughly. i hope yall read it.