Why do a lot of guys get nervous or feel awkward when faced with womens underwear?

Yesterday I was in town because I had a few jobs to do. I bought some Christmas presents, paid in some cheques and then I had some items to return at m&s. I went to returns and it was a young guy who seemed confident. Then when I took out 2 bras and a pair of 'no show' knickers, (that did actually show) he became very embarrassed for some reason. He said nervously why are you returning them? So I said well the bras weren't what I wanted and the knickers despite their name did show through leggings etc. He was red and said okay erm ... as he sorted the return. Once done I went to Victoria secret and boight a few of their cute no show cheeky knickers. That evening I got home and someone I hadn't seen in over 2 years had seen me walk into VS so didn't come over and say hello! I said why not and he said it's embarrassing... maybe it is for guys? We have planned a get together now though so it's all good