Well it is a 20 year old study. Attitudes towards LGBT people were much less accepting back then. The participants in this study are now between the ages of 34-38 most likely. Probably a lot of people in the closet and keeping their relationships discreet or not having relationships, and some that haven’t realised that they’re gay yet.
If you did this study today, there would be a lot more gay sex, but probably less than we’d guess still. LGBT people are still a minority after all.