You have absolutely every right to be worried and concerned about this. I know people hate this name but I'm going to say this. I absolutely fucking hate the 'cool wives' and 'cool girlfriends' on here who are (allegedly) absolutely fine with their husbands being BFFs with some other woman who they don't know themselves. (Often work women who make themselves a BFF to a married man and LOVE the quirky little friendship they have with them.) 
Of course, there's nothing wrong with women being friends with married men, (whether it's single women being friends with married men or married women being friends with married men, blah blah blah.) But sometimes you just know when it's something a little bit more than that, when he's treating you like shit, and you're being excluded from their friendship, and particularly when she doesn't want to know you. You know, the 'oh I just get on soooo much better with MEN' type of woman.
There was a woman at my husband's workplace about 10-11 years ago that worked there a couple of years. They were roughly the same age, and for some reason she assigned herself as his work wife and his new best friend and laughed at absolutely everything he said, and kept messaging him on facebook and 'liking' everything he posted.. and putting 'HAHAHAHA you're hilarious' with LOL-ing smileys everywhere.
When we went on day trips for instance, I'd put 25 pictures on Facebook and tag him in. There would be 10 scenic ones. 5 of both of us, 5 of me, and 5 of him. She was on his friends list, I was not. (As I said earlier, these women don't want to know the wife!) Every single time I put these pictures on, she would go and 'like' and comment on all 5 pictures of him, but would completely ignore every single one of me and the ones with two of us on. (And all the scenic ones.)
When I popped into work with him once and she was there, she was all over him and laughing and giggling, and completely turned her back on me and completely ignored me. She said 'oh ha ha ha careful - we don't want people talking about us, thinking there's something going on,' and gently punched his shoulder and giggled. The woman was fucking 42. She acted like a 14 year old girl.
DH said 'well, why would they say that?' and seemed actually a bit annoyed by what she said. After that he went quite cold on her as he felt she was a trouble maker. But up to then he was happy with the 'banter.' Looks like SHE wanted it to be more than that and was trying to stir the pot. She acted like she wished I wasn't there, like I was some kind of threat. I was glad when she left to be honest.
But yeah, as previous posters have said, it's right that some men can't be arsed to put any effort into relationships and let the wife do all the grunt work, domestic duties, childcare etc. But then will be all over the women at work. And they are absolutely flattered by the attention, and they will absolutely love it if the wife is jealous.
My husband's been like this a couple of times in the past when women have been interested in at work. Likes the attention. (Don't ALL men!) But don't realise how upsetting it is for his wife, and also disgustingly fucking disrespectful. Funnily enough, when any men talk to me or want to be friends with me, he doesn't like it. Up to about 9-10 years ago, he used to say 'who is that fucking ugly loser talking to you?' So he has gotten very jealous when men are talking to me, but it's OK women want to talk to HIM.
One man was very friendly and chatty with me with me about 10 years ago - a man who went to the pub we go to, and DH did not like it at all. OK for women to chat incessantly to him and pay HIM attention though. When this man asked me out for a drink sometime, even knowing I was married, DH was shocked and angry, and he pretty much stopped any friendships with work women from then,
Didn't like the bitter taste of his own medicine!
Anyway, we're fine now. He's not arsed with female attention. And I am NOT arsed with male attention. Fuck that!