Is it suspicious that a married woman have a male friend who she wants to hide from her husband?

I mean it is normal and not bad if a married couple have single friends they both know they hang out with from time to time, is nothing wrong.

But people won't find ok if lets say the hubsand hangs out with single ladies by himself per se or the wife hang out with single guys by herself, it is ok when both hang out with single people at the same time.

So it is much workse when one of them spouses have friends (either married or single) who they hide from their spouse and they dont mention they have these friends. Dont you find it fishy?

Some say possible the spouse hide it from their respective spouse maybe for 1. She or he is cheating on him/her or 2. The one spouse does not get along with their spouse friend (and he /she kows who the friend is anyway) and simply dislike him/her but the spouse still is friend of the person and she/he continues to keep in touch with him/her. so the reason why the spouse does not want his/her spouse to know about the person and keep the friend in hiding.

I found out my sister has this kind of friend, I did nto know about it until last week I discovered it. I dont get involve in my sister private life, that is her business not mine, but I just wonder myself why she needs to hide she has a male friend she does nto want her husband to know about it. I mean I dotn know who the friend is but my sister told me she does not want her husband to know about the guy, cause the husband does not like the guy much. I found out beause during a trip together with my sister and her husband last week to NYC, my sister asked me to buy for her a NYC souvenir for her friend without her husband knowing about the souvenir. So I bought the souvenir pretending it was for a friend of mine and hide it after it in my suicase., just in case my brotehr in law asked me about the gift,

So my sister also told me; When we return to the country some days after I will stop by your house myself and pick up the souvenir"
Is it suspicious that a married woman have a male friend who she wants to hide from her husband?
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