The legs and hair are clearly not for merely a few purposes, nor is the mouth, hands etc.
One leg can be used to scratch another leg, kick someone, etc. It clearly isn’t intended just for walking. Even if we flew…we might have legs (like some birds) to help us sit, or to pick stuff up with our teeth, to defend ourselves in the air. We can’t say, “the legs would not exist if it weren’t for walking.”
Hair obviously isnt just to keep us warm. It provides protection for our skin from the sun and insects, plays a role in sexual attraction, etc.
The mouth not only eats, but talks, licks stamps, etc.
The hands are clearly multi-purpose.
With these things…there is no one purpose which we can say “well…sure there are other uses, but if it wasn’t for this one thing…we wouldn’t have them in the first place.”
Even if we didn’t eat, we might have a mouth still…for vocalizing.
Even if we didn’t walk, we might still have legs…for kicking (in the air).
But with the genitals, even if you can think of other uses, it is clear that if there was no sex…there would be no genitals. One is tied up indissolubly with the other. If we didn’t have babies…there would be no womb. If we didn’t need sperm to reproduce, there would be no testicles.
There weren’t several advantages naturally conferred by the genitals…there was one: sexual reproduction . And if it hadn’t been for that, the genitals would never have evolved/been made.
A similar system is the gastrointestinal: it is for ingesting things. There is no reason for it’s existence other than taking in substances to help the body.
It’s more about the thing’s raison d’etre.
I don’t think we can say, “The legs exist for walking.” That is clearly a major purpose, but we can imagine them existing for uses other than walking. We might still have them even if we didn’t walk, they might still be advantageous and selected for.
But I think it is quite clear “The reproductive organs exist to reproduce” and wouldn’t otherwise.