Mostly because Command Seals unfortunately don't necessarily only appear on the back of the hand.Assigning Rin the responsibility for that is rude and untrue, that was all his own idea and she was more shocked than anyone else.
Issei isn't actually gay despite all the fan jokes about him. If anything we find him asexual, yet sort of also more into women considering he feels embarrased about feeling attracted to Rin and Caster (it's implied his claims on them being vixens aren't just due to him feeling they hide a side of themselves but also because he feels both are sort of devilish and frankly hot. And he, as a monk in training, finds that kind of feeling not good for him).
Granted that's canon FSN Issei. Here things might be different.
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