Red irritation on inner upper thigh close to scrotum smells really bad. i wear under armor boxer briefs for working out.

Sounds like a fungus: They can have a pretty strong odor. Dry warm air, like blow dryer, to the skin there for several minutes at least 2 times a day help prevent the small spores for germinating each day. Topical anti-fungal cream, like Lamisil, (terbinafine) applied after drying. This fungus likes warm moist places. Blow dry throughly and use fungal medication. Good luck. Let me know if doesnt clear.