Just wanna ask this.... when you have kids and you saw your kid reading a yaoi manga what ...

I wouldn't care

Be reasonable about it and act calm

As long as they are old enough, I would not say too much, other than "If you have questions, please talk to me about it". Of course if they were too young, I would be very unhappy and restrict their internet privileges.

That's definitely my kid. It must run in the family or something. I would pretend I didn't see it - wouldn't want to embarrass her/him.

She was sensible about her life choices, so I let it be.

I've already had that happen. My niece brought over a yaoi manga and showed it to my 11 year old. I just took it away and told her it was too advanced for her and that she needed to read shounen-ai or shoujo instead. My daughter loves manga so I have to be careful what she reads.

Ah. Well, since I'm reading yaoi myself, I couldn't really condemn them for that. But my kids are 8 years old. And apart from subgenre's of yaoi (which I don't deem acceptable at such age), I'd rather be worried about pop-up's and related sites.
Ultimately I don't want my kids to be hurt. That's it.
Pictures don't jump and round-house-kick you. But you wouldn't place your child on an "Expert only" ski trail and tell him "You'll figure it out while going downhill". If I taught my kids all they need to know about the Internet and Safe Sex (aside from nurturing their character in its entirety) and they still choose to read yaoi - well go ahead, and hands off my favorites: look for your own!