I believe lesbianism and gayism is more of a psychological and mental thing often more unexplainable and more than meets the eyes the truth is that you do not know how it feels to be a lesbian or gay and actually someone who isn't neither of these two doesn't necessarily understand the psychological understanding of being lesbian or gay, so in my opinion I don't think people choose to be lesbians or gay, infact some people actually started out being straight, but along the line some people became lesbians or gay and some even became transgender.
Being lesbian or gay can actually be caused by peer pressure or having friends who are lesbians or gay if you check the records of people who are lesbians or gay you will see that most of them actually had normal sexuality in their past but as a result of exposure to some certain sexual abnormalities and experiences in their life they ended up switching sex sometimes it could even be as a result of heartbreak that drives people to change sexualism. But I do know that nobody chooses out to be a gay or lesbian it actually starts as a result of so many factors which some of them can be explainable and some others unexplainable.
Sometimes it could be voluntary or involuntary but I do know that nobody was actually born to be lesbian or gay the human system is actually defaulted to the normal attraction to the opposite sex but a twisted crookedness to the mentality and the mindset is the reason why this is so. I don't believe people who actually are gay or lesbian think about how the population of the world would decline as a result of their change in sexual behaviour, in fact this hasn't been thought about and this is very selfish I do believe that if we have so many populations of lesbians and gay that we might experienced an extinction in high rates and numbers of the human race
I don't know a single person who have made a conscious decision to become a homosexual and I know a ton of gay people.
So I suppose there isn't really a problem to be worried about.
As for World's population.
We're already overpopulated.
If by freak coincidence mother nature made everyone become homosexual in the next generation.
Guess what, being gay don't make you infertile, just not interested in opposite sex.
If a gay person wants to procreate, they can do it, provided they find a partner of a opposite sex that wish to go along with it.
Overall, People do want to procreate, gay or straight alike.
In the current world, homosexual couples either hire a surrogate/donor, or adopt, depending on legality of either.
I don't see how that would be different in your scenario.
I'll skip the sexual orientation part.
How would the Earth be populated if everyone is HOMOSEXUAL? EASY, artificial insemination.