OP is saying gay sex being excluded, iiuc. Trend seems extreme since 2008, and it would be nice to see the spikes to 16% or so 3 times between 1989 and 2008. The spikes could be corelated to rising stock market bubbles for all I know. Seems over double the 1989-2008 average that I guesstimate at 12%.

Maybe gay men used to experiemnt with women to “find out”? Maybe harsher economic inequality keeps men home? It’s not obviously internet porn, which became available in late 90s, though 20 years later would get a lot of under 30s.