I can just about guarantee the reason. He knew it happened because the boys told him about it in specific terms and they kept saying "she still hasn't noticed". He has no fear of making sexual comment or innuendo in front of the boys (they are older and DD9 was at the other end of the pool). However, he kept his back to the lady and refocused attention on DD2 swimming. He doesn't really care for the overdone, trophy wife kind of woman anyway so he was probably more repulsed than intrigued. Anyway, it is this kind of thing that makes me appreciate his loyalty.
