The demand for IVF (in vitro fertilisation) and surrogate mothers (surrogacy) would skyrocket.
While there may be less children than now, humans would still have the desire of parenting and raising their own children. Thus they would look for alternative methods which would offer them such options.
For women that would be sperm donations, for men that would require surrogacy or alternatively more traditional adoption. The latter which is, of course, an option for each gender.
If that happened then the world would probably come to an end in less than 100 years because without a means to repopulate then we'd die out.
The human population would inevitably decrease to some extent but, not by as much as you might expect. Most gay people, like everyone else have the instinct and the need to procreate. This would, like @fknmayhem mentioned drastically increase the demand for in virto fertilisation and surrogate mothers.