He told her not to bother, that he sees even his sister in a thong and that it was no big deal. Feeling more comfortable, Kate removed the towel and we offered Doug something to drink. When Kate went in to get Doug's drink he commented how fine she looked and wanted to know if our sex by the pool was really hot. I asked what made him think we had sex and he pointed to the dried cum spot on my bikini. I said "you fucker, did you see us fucking out here?" and he said he had seen it all from the bathroom window. He said it was so hot he jacked off into the toilet while he watched us. Just then Kate came back to the pool with drinks and she seemed pretty cool running around us in her thong. Trying to keep our minds on swimming and not sex we all jumped in the pool.