friends 1 night stand with our coworker

So my very good friend text me last night that he's at our coworker house with another coworker and he's kinda drubk but the night before the coworker who's house he was at made a very open statement saying she wanted to sleep with him and he asked me what he should do I have him advice on doing it, not doing it, and waiting until he was sober well last night he had sex with her and when he told me all I could say was oh because I was surprised. and later I realised I was hurt that he had slept with her....why did I feel hurt? mind you in married and very much so in love with my husband my husband is also friends with this friend of mine. but what do I do?