Sometimes they are seeking only friendship, but more often than not, they are indulging in extra-marital affairs and infidelity.
In some cases, they may be in open, or polyamorous relationships where dating outside the relationship isn’t considered outside the boundaries.
So, take some time to deal with yourself head on and crush that fear of being single.
When you're comfortable being single, you are no longer running back to that familiarity that makes you comfortable and safe.
When we are able to develop this self-relationship, we are able to improve our relationship with others.