So how can there be a God (and I don't mean to turn anyone off with semantics here, call him whatever you want -- Beelzebub, Shaytan, Apophis) when we have sex? Simple: Sex can kill you. That means basically the one good thing we had going for us is forever tinged with "Is this gonna kill me?"
The fact that we have something that's almost purely good but must be forever tainted with worry is just so emblematic of existence as a whole. It's a tantalizingly near miss at something entirely good. Surely, that's a clear sign that a being of superhuman intelligence is fucking with us, and that it wants us to be miserable. Also, the fact that our genitals are right next to our waste disposal systems seems conspicuously like someone's cruel idea of a joke. That can't be the safest or most efficient way for a human body to function. Unless you assume malicious design!
That's why it's a marvel to sit back and behold His grand, terrifying plan. Just don't sit back too long, because you could get deep vein thrombosis and die. Not even rest is safe.