While I was a 20yo student in the UK, I volunteered for a research program the university was doing into sexual responses. It didn't pay much but all money was good for me then. Plus as a shy person, I'd found the nudity and sex-related elements quite exhilarating. So, this wasn't work as such, and perhaps it's not quite MedFet as such either, but after having volunteered, it did put me in a position of pressured humiliation. I was effectively enforced if I was to remain a volunteer. One of the programs explored my brain signals, heart rate, breathing, and perspiration responses to being nude while I was exposed from my chest down to different groups of researchers and volunteers of differing gender or sexual orientation. I couldn't see them, but I was told in advance what each of the three separate audiences would be. I was aware of their presence, hearing them shuffling around as they came and left the lab, their breathing and so on. I found it embarrassing, humiliating, and exciting all at the same time.
After they'd allowed a couple of minutes of settling down time, I'd felt slightly aroused with the hidden all female group of observers, but thankfully without erection response. I'm happy with my size but I'd felt a bit awkward in front of the group of hetero males, perhaps because I was unusual in the UK by being circumcised. However, when it was announced my last group of observers were all male homosexuals or bisexuals I had a shock adrenalin response with rapid increase in heart rate and to my humiliating horror, started an erection. Once started, it sort of became a runaway thing. The more I realized I was getting stiff, the stiffer it got, even though I was willing it to go down. Each 'observation' was held for a few minutes (which seemed a lot longer) but it gave me time to realize that these observers would never know me, I'd never know them, and this is presumably the sort of thing the researchers were looking for. It was a genuine natural response for me, so who cares?!
Apparently, my response was typical of other males who identified as predominantly heterosexual - a category that they concluded commonly showed significant bisexual responses (in females too).