My wife is also using testosterone cream so we will monitor her labs and see if there are any unusual spikes. It seems to me like the hydrogel dries pretty quickly. I guess if we saw evidence of transference I could try applying cream only to the rear side of the scrotum or wait longer between application and sexual activity.
With my daughter, I am not really concerned. I apply the cream with one finger that I wash very thoroughly afterwards. I thought the toilet seat might be an issue but now that I'm paying attention, neither my scrotum nor the part of my inner thigh adjacent to it ever contacts the seat. I think some guys have mentioned they worry about scratching their balls and then touching things, but that is just not something I do -- I find it very easy to avoid touching my cream-laden nuts with my bare hand throughout the day. If some residue remains in my underwear, I very much doubt it would survive a wash cycle with laundry detergent. So, I am not seeing any realistic scenario where transfer occurs.
