If you ask me how I answer for myself: I like good sex, and early sex is, in my opinion, not good sex. (It’s all fumbly and awkward— not at all the hot and heavy hook-up we like to imagine.) So if I invest in sex with someone once, I want them around long enough that that sex actually gets good, and I’ll wait long enough to get the indication they will be. (This varies, but it’s definitely longer than three dates.)
They couldn’t tell the future; couldn’t promise it’d be what you both hoped it would be. They couldn’t swear it’d work out, or they’d never leave. They couldn’t assure you they wouldn’t break your heart or otherwise fuck you over — however inadvertently — and they didn’t want that for you.