OrphanWilde also made an example about writing Web pages, whose opportunity cost is much higher than for reaching stuff but much lower than for having sex, so there might be more of a continuum than you realize.

I can point to a conversation on these forums where a woman stated she had pity sex with guys, for pretty much the reasons I laid out - that's what pity sex -is-, it's having sex with someone out of that sense of obligation arising from a perceived social advantage. It's not like I took an entirely fictitious example and presented it as an analogue of real-world things, they're all real-world things, and they're not the only ones.

You're stating that they're different, but that's from an objective outside perspective; from the inside obligation feels like obligation, regardless of its reasonableness.