They Have Too Many Friends Of The Opposite Sex
While these days it is normal to have friends of the opposite sex, it can be a cautionary sign if your partner has too many such friends (and too few same-gender friends). The reason for this is simply statistical – the more people of the opposite sex your partner hangs around with, the higher the likelihood that at least one of those friends is going to try to flirt or otherwise show signs of attraction.
It is normal to have friends of the opposite sex but what is not normal is when they have new friends and talk all the time about it. It can be a colleague at work or their gym instructor or anyone else. Even if they don’t have the intention to cheat there might be a situation where you’re not around and they are drunk and one thing leads to another.