We don't have cellphones, so maybe my comment is worth nothing. However, my husband is also younger than me, just like yours. There is one young female in his office with a few other guys. It's obvious he has no interest in her, and in fact I feel funny even thinking about it. We are both capable of feeling irrational jealousy for a moment, but that is it.

I don't blame you for feeling suspicious. An explanation afterwards is not enough, to me. Do you *really* feel the afterwards explanation is adequate? I can't help thinking there is some reason you have doubts. Not that he's doing anything wrong, but maybe there's something right he could be doing.

You guys need to talk it out so afterwards no explanations are needed, so that the initial behaviour is comfortable. Just my two cents, because I couldn't live with what you are going through. (Again - not blaming either of you. Communication resolves this kind of issue, because you should both want to live without this pattern.) I study with guys from school (my field is male dominated) and have encouraged my husband to do a ask the girl in his office to work on a project with him. There's nothing uncomfortable about that. It's not like we're going to a bar with someone of the opposite sex or flirting with them or something. Draw your own boundaries, together, and if they aren't comfortable, talk again and move them. Well, good luck with this.