I'll post some specific personal examples in the hope that other guys will too:
The one night stands that you have who seem to go all distant after that one night of crazy dirty sex were on break from their boyfriends for the night. They are disappearing because they have screened you as being good for sex only, and they are worried that if anything long term goes on then they will get emotions and ruin their relationships. Or perhaps you just hadn't convinced them properly (using subcommunication) that you could keep it all hidden in the longer term.
Girls can enact ridiculously complex plans and organise for logistics and the type of sex they want with you even before you have left the bar. One girl "lost" her phone and "couldn't remember" her own phone number yet still came home with me and wanted sex all night. In the morning she was gone, never to be seen again. My best guess is that she either had a boyfriend or a child or both, and therefore planned to go ghost right from the start.
Even if you're the biggest "alpha" in the world, it's still important that your women sleep with other guys in order to satisfy their sexual and emotional and social needs. If you tell them they can't then they will anyway. It may be that telling them they can actually makes it less likely... not that that matters in the slightest. If you don't understand this then you haven't fully digested the red pill. Read "The Selfish Gene", "Sperm Wars" and "Sex At Dawn" for details and then go out into the field non-stop for six months.
A girl angling for monogamy when it's clear from your actions that it won't fly has suddenly found an acceptable backup guy or has decided to finally settle down with "that friend I've known for years". She is trying to get more from you because her bargaining position has improved. If you don't f*ck it up (for example, by getting annoyed or emotional), you can still sleep with her when she shacks up with her new guy.
The most important thing in the game is not being "alpha", but the ability to keep your f*cking mouth shut. You need to be friendly all night to the boyfriend of the girl you just f*cked and the guy has to *love* you. If you act all "alpha" and compete with him then he will get pissed off and the girl will be gone so fast your head will spin.
"Provider" strategy guys subconsciously know that "friendly" strategy guys who are friends with their girlfriends are either sleeping with them or have slept with them. Their genetic programming and therefore their subconscious knows that their girlfriends are going to sleep with other guys, and they would rather it not be with an "a$$hole" strategy guy who is going to try to take their girlfriend away from them. Sometimes they rationalise it by believing that you are gay, which is fine.
Sex means a hell of a lot less than you probably think it does.