Bingo. Also gj on finding it so quickly, I just realized what I wrote is probably summary of at least 50% of manga and dj that can be tag with "vanilla" and "succubus".
Huh, that's... I don't know if I should call it a good or a bad joke... Dunno, really. It's an interesting joke I guess.
Without context I can only guess, but I assume she started dating the man in question, and he didn't know she was a single mother.
I'd assume they were on the stages of getting to know each other... As in flirting, but with something in mind to see if they should start dating. Being on flirting stages seems more likely than actually dating I guess.
You're supposed to say it's rage-enducing. More info: http://knowyourmeme.com/memes/events/pineapple-on-pizza-debate