COMMUNICATE both verbal and non verbal and a good time will be had by all involved!!!
I'd like to reinforce what LB is saying here: Good sex requires communication, even if he's your BF/partner and you've done it together a thousand times. Although I like porn (don't we all, or we wouldn't be here!), I think it's unreal to have guys screwing away with no communication about what they like/want/don't like/don't want/etc.
As a top for the last ten years who started out as a bottom, I think the guys you meet who have loose, floppy arses that don't stimulate your cock are probably trying too hard to relax–they've mastered that [relaxation] technique very well, and just need to be asked to clamp down a bit. After all, they are able to tighten up when avoiding a bowel movement!
For guys who are "too tight" and in pain, they're on the other side of this situation: They need to learn to relax their sphincter. But this is not intuitive for many guys–relaxing the sphincter is usually associated with taking a shit, and that's not something a guy--especially a novice--wants to do in the middle of sex. But if he's empty and clean inside (which we hope is the case), nothing will come out. I remember one college guy who had so much trouble relaxing this way that I led him into the bathroom, had him sit down on the loo, and try to shit. When he saw that nothing would come out regardless of how much he relaxed and even pushed, he was more confident about relaxing in bed and we had a great time.
A final note: One time I hooked up with a bottom who had a genuinely small arsehole. He relaxed completely, enjoyed immensely getting fucked, but it was so tight around my cock that I'm the one who was in mild pain--I kept putting on more lube, but it was still too tight. I was happy when he finally came so I could pull out, and then he finished me off with his mouth. But this guy is the exception rather than the rule, and no, I don't still have his phone number 😉