my opinion:
I don't think people should engage in sexual intercourse until they are ready to have a child/unless they want a child. Paid sexual services (prostitution) should not exist, and condoms should be invented in such a way that they protect STI's, but do not prevent pregnancy (i dunno how thats gonna work, but i want it to happen).
Reproduction is natural, and so sex should be as close to natural as possible...meaning the process should go ahead, and end up with pregnancy. Preventing STI's is fine, as ever since man began, we've been trying to find cures/remedies/ways to prevent illnesses.
That being said, condoms for masturbation is fine...less mess.
In short, why is there the need to engage in sex so often? Ever since the first two humans reproduced, we've lived with doing it without condoms...If you reach the desired number of kids, then stop engaging in sexual intercourse.
Sure sex is good, but there are other good things in life...oral feels good, masturbation feels good, mutual masturbation feels good, massages feel good, eating chocolate feels good, exercise feels good, listening to music feels good, even video games feel good.