Thank you for your question. Many guys wonder about this, so you are not alone. The good news is there are no health risks to ejaculating in your underwear. The downside is your underwear will feel wet for a while, and then it will likely feel a bit stiff (like a starched shirt) when it dries. The stain and stiffness comes out when you wash them. Many guys find it easier and more comfortable to use a towel when they masturbate but ejaculating in your underwear is fine too.