He prefers wearing as few clothes as possible when not in battle gear, at most he'll wear a tank top and jeans. More often he'll simply walk around in just some shorts. He knows he's is very good shape and that his body draws glances but he doesn't care. The Scions have just learned to live with it

Estinien on the other hand is much picker about his partners and doesn't like the idea of sharing R'ythian at all (he's learned to accept Thancred). The only person he might be okay with is Aymeric, but even then...he doesn't see him in that way

It's very similar to his relationship with Aymeric, another person helplessly infatuated with R'ythian whom he refuses to let get too close. The sad irony is that both of these men are actually very aware of his faults, and still want him despite them. Maybe someday R'ythian will get over himself 🤷🏽

It's not that there's isn't any mutual attraction, but rather that R'ythian is afraid that G'raha's idealization of him blinds him to all of his flaws, and that if he ever got any closer to him, he'd realize the real man is much worse than the one he thought he knew.