Regarding the reliability of the "gut":
A prerequisite for ongoing participation on this site is that you have experienced infidelity. Therefore, the vast majority of us contributing have probably experienced the period of doubt and questioning before having our d-day where our "gut" feelings were confirmed.
Therefore, most of us here can firmly state that "our" gut was correct.
However, that does not that every gut-call for everybody else is correct, or even that the gut is a reliable sensor to confirm infidelity. What is missing from the equation is the number of spouses who had a gut-feeling that then proved wrong.
It’s like if you were to ask a group of 1000 kids from all over the world to draw a bear. I’m guessing 998 would draw a black or brown bear because that’s what they know. The two kids from the Arctic would draw a white bear and be perplexed about all the other pictures. Ask those two and in their experience all bears are white.
Ask us – and all gut-feelings turned out correct.
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Your last post rings a couple of bells.
I experienced infidelity in my first really serious relationship. A 5-year relationship with and was engaged to marry four weeks and six days before walking in on her and another man having sex. I left that relationship and went through the terrible months of pain and self-doubt and all that. About 18 months later I met my present wife and have been with her for over 30 years.
About 12-14 years into our relationship, I was convinced she was cheating, but didn’t have the smoking gun. I posted on infidelity sites (like this) and the overwhelming tone of the replies was that yes, she was definitely 100% cheating. Don’t recall a single voice suggesting I investigate better or that maybe this wasn’t cheating.
The "red flags" I had were issues like less intimacy, lots of time at work, irregular hours, arguments, time at the gym, changing wardrobes and hairstyle and whatever. Each item met with a flood of "yes – that is all the proof you need. A new perfume is a definite 100% indicator she’s cheating".
What got me thinking critically was when I saw a behavior defined as "irrefutable proof", and then when that behavior was reversed it was also "irrefutable proof" of cheating. Like... lack of intimacy is proof of cheating, but if she initiated intimacy it was also proof of cheating. New hairstyle was proof of cheating, while not taking personal care was also proof of cheating. I realized that there was no way to win with that sort of logic.
I went back to my police training, and after var in her car, tracing financial spendings and all that I realized our marriage was in trouble, but not due to emotional or physical infidelity with someone else. We had the typical issues of growing apart and not working as a team and I had to admit that a LARGE part of those issues were due to the distrust I displayed due to the infidelity I went through in the previous relationship. I dealt with that – I got professional help to deal with my PTSD from law enforcement and infidelity. We then found ways to improve our marriage – to grow closer. Resulting in what I would consider a very good, mutually beneficial and respectful relationship.
I have NEVER denied your wife MIGHT be having an affair. All I have said is that your evidence is thin, and that you should investigate for the TRUTH. That truth might be that she’s offering sex to anyone for free coffee or that she had sex at the conference and again at the office. Or it might be that she’s in the early stages of an EA (although considering your doubts about paternity that would be a very unique "EA" so directly after a sexual encounter leading to pregnancy). Or... it might be pent-up issues due to your past experiences...
What I will say – and quite forcefully – is that suggesting you divorce and automatically deduct she’s having an affair simply because your tummy is giving you gas is IMHO really bad advice.
"If, therefore, any be unhappy, let him remember that he is unhappy by reason of himself alone." Epictetus