In theoretical discussions of parallel universes or alternate realities, the concept often involves the existence of multiple versions of oneself existing simultaneously. However, it's important to remember that these discussions remain in the realm of speculative science and are not currently supported by empirical evidence.
The question you pose regarding having sex with oneself in a parallel universe is a matter of interpretation and personal perspective. The concept of masturbation typically refers to self-stimulation for sexual pleasure. In the context of a parallel universe where a version of you exists as a distinct individual, engaging in sexual activity with that version could be seen as a form of consensual sex between two separate beings rather than self-stimulation.
It's worth noting that discussions involving hypothetical scenarios or philosophical questions about parallel universes can be intriguing and thought-provoking, but they may not have definitive answers since they are based on speculative concepts that are beyond the current scientific understanding of the universe.