He's embarrassed because, erm "something else came out". It's not blood.
Think it was implied that he “squirted”...
(I didn’t think there was any subtle way of saying it... for some reason I feel the need to apologise...)
Not sure why I didn't want to type it either, I mean it what it is. (▰˘◡˘▰) I'm guessing because this manga isn't presented as being "smut" or anything particularly vulgar, an incident like that, especially giving the context, where they are forcing themselves to remain emotionally distant, showed more vulnerability than Jaeha felt comfortable with.
http://www.mangago.zone/read-manga/lover_boy_lezhin/uu/lover_boy_lezhin-chapter-30/9/
so here jaeha is asking eunho to stop or slow down and then something happens that causes jaeha to cry. is he bleeding when eunho says "i didn't know that such a thing is possible for men"? i just wanna clarify, sorry if it's painfully obvious to everyone but me haha