Male Speedos for SDXL is a simple lora for adding detailed speedo swim briefs to your generations. To prompt for it, use "wearing a speedo" or "speedo" or both. It tends to prefer blue colored speedos, but has no issues adapting to whatever color you prompt in my experience. I have not prompted for color or style in my examples. The keyword "bulge" was trained, and does have a small effect, however I didn't use it in any of my examples.

Current issues are lack of data from behind. You will get a lot of bare behinds before you get a decent one wearing a speedo.