You meet someone and you fall in love and that's that. To quote Woody Allen: 'The heart wants what it wants. While they're not blood relatives in any sense, they still fall within the definition of incest.
I've seen similar stories where random married couple's in-laws have fallen in love and ran off together to be married etc. I also wasn’t comfortable with lying about who I was or who I loved. I totally rejected the idea that I should hide how I felt, as if it was wrong or horrible. To me, coming out was all about being true to myself.
You also here stories about long lost brothers and sisters who are married with kids before they realise (Can I change the spellchecker on here to U.K English in any way?) anything. I came out at 16, shortly after I discovered I was gay. I'm not saying it should be banned, just playing devil's advocate.
If two related people want a baby together, something needs to be done to prevent that from happening. But due to the problems with incestuous conception it is also a problem, especially being that no sexual contraceptives are 100% effective.