trying to prove a point

So my boyfriend has a dog, and she hasn’t been bathed in at least a month. the front and back yard is all dirt, and she’s constantly rolling around and digging in it, if you pet her for like 30 seconds, your hand has a layer of dirt. we were about to take a shower, when he started playfully spanking his dirty dog with his clean underwear. I complained that’s gross, considering those will touch his privates, and his defense was that the inside of the boxers touch him, and he was spanking her with the outside. Am i right for thinking that’s gross, or in the wrong?