Why is this relevant to if something is good or not? Why is "nature" inherently good and any supposed deviation from that bad?It's weird that simple biology confuses you. Yes the lesbian consciously just wants to get off. But the only reason we want to get off at all as humans is to reproduce. It's like saying we only eat because food tastes good. No. Food tastes good because that's our brain's way of getting us to do something that is necessary to survival. The only reason we find sex pleasurable is because its a way for our brain to get us to want to do it.
The true purpose of sex is reproduction. Not busting nuts.