Yes, you're totally right! Besides cock to cock frottage, which is out of this world, one guy either sliding his cockhead or his entire cock shaft up and down the other guy's ass crack is amazing and ultra erotic, either in missionary position, or even better, doggy position. It's like you're fucking without penetration. Both of you feel the sensations of cock against ass, including against the asshole opening--almost intercourse, certainly with the pelvic thrusting--and it is beyond wonderful. I've enjoyed it myself a number of times. If you work up a sweat frotting/fucking each other in this way, you can use the sweat between the ass cheeks as lube, and really go to town. Plus it's bare cock against the ass because a condom won't be needed without penetration, and those skin to skin feelings are awesome. And even if cum explodes out of the cock and all over the other guy's asshole opening, it's still safe, very very erotic, and an incredible pleasure. OMG, isn't it awesome how many ways 2 men can pleasure each other?!
I have done both types of frotting. Cock to cock stroking was such a surprise as to how warm his feet against mine! And me being on top, rubbing and sliding in his cheeks, skin to skin and against his hole felt awesome, as you said! I look forward to doing that again! And actual raw fucking once on PrEP and DoxyPEP!