What is this suppose to imply…?

That she wasn't this person's first choice of fox girls to sex.

A popular thing in the Japanese Touhou fanbase (haven't seen any westerners really talk about this) is Yuuma being Ran's ex-girlfriend who is still sad about their breakup, and the whole thing of forcing Tsukasa to be a substitute for her is a meme that spun off from that.

I get the "mean" part, but where is it supposed to be hilarious?