Ok, I need some serious advice here, and for the sake bias I will remove all references to sex...

Say person A and B are in a relationship for over a year, live together and have their kids living together. Person A is unemployed and person B works full time. They both agree in their relationship that having outside friends of the opposite sex is unheahlty for their relationship. About a year after being together, person A secretly breaks that agreement and begins talking to outside people of the opposite sex (more than one), but specifically one person (person C). Person A's son also all of a sudden begins wearing T-shirts from person C's company and person A keeps frequenting the business to "get things fixed". Supposedly person A claims they knew this person C for ages and is considered a "cousin" even though person B never met person C in over a year of dating. What makes it odder is that person C is married and has kids and person A has not met the spouse of person C nor has the kids ever met or did anything together(although supposedly they are like "cousins"). The ONLY connection the "cousins" have is via mutual friends/family (not blood) and it seems to only be restricted to themselves.

Obviously something was amiss in person A and B's relationship, so person B checked the phone logs and low and behold, there were constant voice, text and pics sent back and forth, many first thing in the morning and last thing at night (good morning/good nights?). When confronted, person A claims they were only talking to get the opposite side sex's opinion on problems that A and B were having.

So, in an effort to trust person A, person B suggests that they ALL (persons A, B C and C's spouse) go to dinner and sit down and lay it all in the table so the truth comes out and there is nothing "hidden". Person A IMMEDIATLY refuses. Thenlater claims that he/she only refused beause he/she did not want to get other people involved in their problems. About 3 months later, in discussing the issue again, person A reveals that person C was "always" at person A's aunt's house, a place where person A would go but not allow person B to go because he/she claimed it was a "messy" place and that he/she was embarassed by it.

So, now I (person B in case you got lost) feel like there has been something going on all along and that I am a fool for even believing in person A to begin with.... am I wrong for feeling there is something going on?