One on one time can be an important part of fostering a healthy relationship for some people (as pretty much everyone has already said). It sounds like it isn't something you connect with, which is what it is.
In my world, possessiveness over time with someone I care about is not healthy. I like to compare how I feel about my lovers to how I feel about my friends. If I'm being envious about how much time one of my friends is spending with someone else, that's immature on my part and something I need to deal with. If there is some kind of gathering with my friends to which I am not invited, that's their choice; not every event is perfect for every dynamic of guests. I don't own my friends, they don't owe me anything, and they are grown humans who live their own lives (why would I want it any other way?).
Your husband, of course, can be "not thrilled" with you spending time with someone that doesn't include him. This is, however, his own issue to deal with. Grown ups spend time in different groupings depending on what they are doing. This is a good thing.
Me: male, 40, straight, single