Unpopular opinion I guess but i do recall reading that with a regular partner a woman's vagina does change to fit that particular member. A minor change to be sure but from what I recall it will sort of adapt to size and shape to increase pleasure. I suppose this makes sense in the primal aspect. Sex for reproduction so you want your consistent partner to orgasm consistently. Not saying multiple partners makes it unpleasant. That sounds like super bullshit. I could provide sources but to be honest it's been a long day and I thought this was common knowledge. That being said I've always felt the first time has room for improvement and this could be a factor.