Why do my friends try to hide their affair from me?

I'm pretty sure two people in my circle of friends have an extramarital affair. Both of them are married with kids and one of them is currently building a house for him and his family and he always shows his affair pictures of the progress. I noticed them sneaking away quite frequently and once walked in on them talking to each other in baby voice. Recently I feel like theyve been keeping an eye on me (looking at me from afar, looking at me over their shoulder etc.) or sneaking of when they know I'm distracted etc. Why would they try to hide it ONLY from me and not from others?

Why do my friends try to hide their affair from me?
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