Soliquid but they're really into marking, blood, and bruising. That's part of why they constantly fight. It gets their blood pumping and their dicks hard.
Liquid constantly trys to pull one over on Solid by palming at his erection or grinding on it when they fight.
Also, I don't know if you think either Solid or Liquid are trans, BUT HERE ME OUT. Transman Solid who doesn't have his t-dick yet. Liquid knows this because he's a pervert and is obsessed with trying to dick down his brother and impregnate him.
Liquid wants to establish dominance over Solid so bad. It just feeds into his power fantasy.
Ty other anon for making me think of Soliquid.
― Bosscest anon
Soliquid has TOO MANY possible fucked up scenarios. Sorry honey that I will repeat myself, but 99% of people here isn't in my Discord server so- Liquid 100% has a "gene" fetish, he passed years thinking about his "twin with dominant genes" and the more people kept belittling him for having recessive ones Liquid BOTH became angrier and hornier. I'm pretty sure he jerked off to both fantasies where he's the winner who forces his "dominant brother" into submission and to be the one who's forced into "his right place" ajsnxonaowns (using a lot of "" cuz we all know Liquid's genetic knowledge is SHIT; mostly because it's literally just a fetish to him lmao).
The marking part, still I think it's more a Liquid's fetish (oh gosh he's such a weirdo-), like, they're fighting and Solid, even if half distracted by the Muscular Man Without A Shirt On factor, still fights seriously, while Liquid subtly makes it more sexual on purpose and he barely notices the shift until they're growling and biting and grinding on each other on the floor and cumming.
ANYHOW if anyone is interested in my personal genitalias headcanons, here's a thing I did once at like 3 am or something.