Well, this was an interesting thread to wake up to.
That being said, I feel like this is an issue where I'll say one thing here right now, not actually being in the situation, but if it were to truly happen to me I'd react entirely different.
I can't pinpoint why, but I feel like this is something that is different than your children simply revealing they have a differing sexual orientation than you expected. This is revealing that they have a different sexual orientation towards each other . I can't honestly say if I wouldn't sit down and try to figure out if it was something that I did. Like people have said, the human mind is a complicated thing, making it a possibility that something I did, whether how I treated them or maybe how I let someone else treat them, made them attracted towards each other.
However, present rational me on this thread is saying that while I'm sure I'd find it incredibly shocking to say the least, I'm not sure if I could really do anything. To be honest, at that young of an age, I don't believe I'd facilitate those feelings. Sure, I can't get rid of them, and I'm sure I wouldn't try to, but as smart as some 14-16 year olds are, there are many that still don't know what they want in life. I'd say I would have to do my best to express that I don't want to appeal to this action, but I wouldn't do so in a way that would repress them.
Once they are adults however, my job is done. If they decide they still have feelings there is literally nothing I could do, but I'd at least feel better that they are old enough to have had time to think about it.