They can adopt kids, yes.

According to wikipedia, 9.4% of gay couples have kids. I dunno what percentage of hetrosexuals have kids, and I dunno what the average age of gay couples is, but it looks like gay couples are a lot less likely to have kids. This is understandable, since people want to raise kids which are related to them.

So yes, my advice would be that bisexuals should only get into hetrosexual relationships, unless they are both ok with sperm banks/adoption. Incidentally, according to some people, the majority of bisexuals are only interested in hetrosexual relationships (and gay sex) although I don't know whether this is because they want kids someday, or because they are hetroromantic.