Ever heard of surrogates? If the only problem is keeping population up (humans are quite good at this) then they can just make a law legalizing surrogacy. I bet you'd see gays availing that. Lesbians have it easier, then can just get a sperm injection. Many homo couples do want to have kids and they're adopting - adopting babies that straight people are throwing away.
Surrogacy is getting more and more difficult nowadays. Recently surrogacy was outlawed for foreigners in India. Many other third world nations are also following this example. Right now it is legal only in a select few countries, such as Georgia and Ukraine.