It sounds like perhaps things had been much more frequent sexually, and then his being intimate with somebody else came at the end of a "dry spell" for you two, so it seems scarier and like a portent of things to come? I'm going to include a lot of details about my own relationship with my husband here because as you said, not a lot of people here post much about libido differences between them and their male partners.
When we decided to open our relationship again, we'd had many sexless 2 week stretches, even a month once, and I was terrified that him having another sexual partner would mean that would be the standard. I really struggle with not having sex at least once a week, sex is the only big issue in our relationship but it is also one of the most important things to me in a marriage. After going to a counselor for this issue and it not improving, we spent a couple more years struggling with our different desires. At that point, it was time to open the relationship or talk about separating. Things have evened out at about once a week, with me doing 95% of the initiating. I haven't had to face whether his having another sexual partner would affect our sex life - both of his relationships that got to the point where he'd finally decided to have sex with them, ended before it happened.
Perhaps you could step back and take a longer look instead of being in the immediate "this feels crappy" place. Decide to watch how it plays out over 4 or 6 months. Discuss and have a fallback plan for what you each want to do if it turns out him having a sex partner is a permanent negative for your own sex life. My husband said if he starts having sex with somebody INSTEAD of me as opposed to AS WELL as me, it wont be a positive for our marriage and it means he isn't in a healthy place to have other sexual partners.
Do you think it feels different for you in these two scenarios?
1. He didn't have sex with you for 2.5 weeks then had sex with her
2. He had sex with her then didn't have sex with you for 2.5 weeks
For me those two difference scenarios bring up different feelings, each gives me new things to look at about my own self esteem to work on. Is it possible he had to work up courage to have sex with somebody new, and was that distracting him and making him nervous so your sex life took a beating?
We saw the sex counselor, about 5 years ago and I think it made things worse (mainly because I started going with him and I saw her give him advice and him not follow it, which was really frustrating). As things are pretty much in the same place, he has just started going to another counselor (we are lucky enough to have a highly recommended local poly friendly counselor). She is going to be tackling his issues from where they should've probably been tackled from in the first place, emotional stuff instead of what his penis is doing. Off the top of my head he's got incredible work stress, he's never felt he really deserves to be happy, and gets really sad when I express that I'm unhappy because he isn't having sex with me instead of finding proactive ways to improve the situation. Mix performance issues in with that, it isn't much surprise he avoids sex.
I find it likely your husband probably has issues that contribute greatly to his sex stuff, so perhaps you would think about going to different counselor to tackle it from a different angle, it's pretty hard to have a healthy libido if you don't love yourself enough to let yourself be happy.
Happiness will never come to those who fail to appreciate what they already have.