Not because of obligation or "supposed to" but i've definitely had one sided sex because either me or my partner wasn't feeling it or was on their period or had a headache or cramps or was tired or something. This has happened with my current Miss and with past partners. It's just something that happens. i've never felt like it was any kind of obligation or anything and don't think any of my partners ever has either. i actually see it as kind of a benefit of being in lesbian relationships. Often sex is one and then the other, simultaneous activities are more of a special occasion thing, like strap-on sex or 69, not something we do every time we have sex. So when one of us isn't feeling it, it's easy to just do half. Shit happens. Sometimes one of us is too tired to reciprocate but happy to let the other pleasure her and it's never a burden for the other. Sometimes my wrists hurt, i have carpal tunnel, and using my hands can be painful, or one of us is having allergies and has a stuffy nose so oral is hard, so that night might be the one who isn't having some kind of handicap pleasuring the one who is. Or one of us might be too tired to have an orgasm (they can be work sometimes!) and is happy to just take care of the other, or one of us has a headache or something and doesn't feel like having the tension and build up of an orgasm that might make it worse. It generally works out and neither of us minds when we don't BOTH have an orgasm because it's still enjoyable for us both anyway. We can enjoy the activity and the intimacy without the orgasm.