This happened to me. While I was dating a guy (we were together for 3 years) one of my friends always used to comment about how he was so good looking, should be a model, dressed really nice etc. She made these comments on numerous occasions so I kind of figured out she liked him.
Around the time we broke up, I bumped into a mutual friend of mine and my ex's and he was asking me what the deal was with my friend, he thought she was crazy and went on to tell me that she had met them on a night out and went home with another friend of our's, my ex was also there on this occasion.
The next time I saw her she was telling me she wanted nothing more to do with the guy she went home with, the next time I saw her after that she told me she was pregnant by my ex and that he was denying the baby. She never mentioned that she was hooking up with him, but thought I needed to know this (LOL).
Luckily for her my ex and I were still good friends so I called him and asked what the deal was. He claimed it was a one night stand and a week later, she calls him saying she was pregnant, he wasn't having any of it, he also commented on the fact that she had also slept with his friend, he was like who does that? As far as he was concerned, if she was really pregnant, it could have been anybody's.
I talked him into accepting the pregnancy until he could prove otherwise, I know he's not that guy that would not want to be involved in his child's life or do the right thing. When the baby was born, he looked just like my ex so he fully accepted the child.
Every time she had an issue with my ex she used to call me, I had to cut her in her tracks, 1. not my business, 2. you can't expect me to be getting involved every time something goes wrong, 3. it's not fair to put me in the middle (I was in a relationship with someone else at this point) 4. this was obviously something you wanted so handle your business!
To answer the question, it would depend on a few things, if I were on good terms with the ex and they were open about it then no, I wouldn't have a problem. If it was an ex where the relationship ended badly, that I didn't have no contact with it would bother me a little as it would have an impact on the relationship I have with the friend, I would also question why my friend would want to get into it with one of my ex's, after all he's my ex for a reason right?
Bottom line I think it would be the secretiveness that would bother me the most, why would you need to hide it?