Sex and the future

Devon and I were watching an absolutely wretched episode of "Star Trek: Deep Space Nine" last night that sparked a debate about holosuites as sex toys.

In that ep where Odo and Kira finally hook up after wasting most of the episode listening to lounge music in the holosuite, I wondered what freaky things people would do in there if Trek were more like real life -- or even more like "South Park."
Moral question: Is having sex with a hologram cheating, or is it more like using a super-advanced sex toy? Devon noted penetration as the benchmark, but many toys are designed for that, and lesbians can have sex without it, so we'd have to start by defining "sex." If virtual reality advances that far, will marriages break up over illicit Jacuzzi time with fantasy people? And how does this impact the concept of a threesome? Is it truly a threesome if there are only two biological life forms there? And what if the hologram is programmed to look like one of the other two participants? Is this a creative solution to a problem or the height of narcissism?