A girl is three things: the unconscious id, the ego and the superego. The unconscious id decides if she is attracted to you. But the ego and the superego must agree (unless she’s drunk or something). The ego is concerned with her objective interests - it might disagree if you have objective flaws, like say being notoriously unfaithful, being an asshole, being known as bad for sex, or being broke, or if having sex with you will deteriorate her social standing. The superego is concerned with social rules - it might object if the sex breaks the social rules she considers important, like if you are married, if you are of a certain race which is despised in her family, if you are not respectable in a way or another.
If the ego disagrees, it can be convinced using rational arguments. The superego can be overcome if she has enough attraction to you (some girls have weaker or stronger superegoes - in general strong superego is good for LTR but bad for sex)
As far as one-night recreational sex is concerned, the main obstacle is usually the superego (‘sex without love is dirty!’). It can also be the ego worrying that ‘sex will not be good’ or ‘people will think I’m a slut’.
If the ego disagrees, it can be convinced using rational arguments. The superego can be overcome if she has enough attraction to you (some girls have weaker or stronger superegoes - in general strong superego is good for LTR but bad for sex)
As far as one-night recreational sex is concerned, the main obstacle is usually the superego (‘sex without love is dirty!’). It can also be the ego worrying that ‘sex will not be good’ or ‘people will think I’m a slut’.