Much like the use of her old N7 armour, it's irrational and… odd, because he's not programmed to desire her, to desire the warmth, the scent, the softness... though Shepard's body is only soft in comparison to his own. Her mouth is open and he can taste her tongue, though he doesn't have one himself. He can't kiss her back, so he caresses her skin wherever he can reach and runs his fingers through her fiery red hair. His hands are the only parts of him that even come close to a decent imitation of the humanoid form. And frankly, the best tools he has to work with in this situation.
"Shepard-Commander…"
She breaks the kiss. Her cheeks are flushed and her breath is hot and heavy, but far from oppressive. "I think we're past formalities, Legion."
"Why are you doing this?"
She sits up and straddles him, leaning forward so that her breasts peek out of her overalls. "Curiosity."
She's grinding against him and it's nothing short of amazing, but he's got nothing down there but erogenous sensors and really hopes she's not expecting something else. He tries again. "Shepard-Commander… we do not work the same way as organics."
She strokes the antenna-like attachment on his left shoulder. "It's the differences that excite me." He understands this perfectly; though it never occurred to him she could feel the same way.
She keeps stroking him, and her touch makes him tremble between her legs and in the wired tips of his fingers. The vibrations don't stop and then she realizes it's something built into him, and a string of gasps follow.
This morning her palm grazing his chest on accident had seemed to be the height of physical contact. Now she's peeling off her clothing and putting his hands on her bare flesh, letting him explore her outside and in. Her warmth and wetness rub off on him until she's moaning and screaming and he is too, just not aloud, and he knows what he's doing to her is just as good as if he were anything else.
She wraps herself around him as tightly as possible and lets out one last cry, and suddenly it feels like he's breaking down and powering up at the same time. Shepard's hair stands on end but she's not in any pain. Finally, he lies still, but she still clings to him long after, shuddering and panting.
Her heart beating against his chest almost feels like it could be his own.