Re:why dose Xev refuse to put out for Stan?

Xev is only partially a love slave.
The drive to fall for anything male that moves (and I was going to say ‘and is still breathing’, but that’s obviously redundant in this case 😆 ) was transferred to 790. Not to her.

She simply doesn’t find Stan attractive and therefore has no drive to ‘put out’ for him.

She does later in series 3 become good friends with him which is quite touching.