There could be a lot of reasons someone loses interest in intimacy. Some of the more obvious are depression or a lot of stress. Depression can make people lose interest in a lot of things they enjoy or even to the point of self care. Some of the signs of that are a person withdrawing from people, even loved ones. Them losing interest in things they used to enjoy. Them having a negative outlook. When it gets really bad, they may stop working, barely eat, lose all motivation for simple things such as personal hygine, sleep excessively or sleep very little. Possible self medicating with drugs or alcohol.

Outside of depression it could be a lack of caring for a partner. Sometimes we get in an unfilling relationship and aren't happy, but we don't want to be alone so we stay even though we now we aren't going to be happy. Maybe this is for monitary reasons, or self esteem, like thinking I can't do any better. The relationship is fulfilling if you and your partner have the same goal, can be open and honest and treat eachother with respect. All of this while moving foward in life and accomplishing personal goals and mutual goals. Such as work, school, having children, mariage. Can you answer the question, "Where does you significant other want to be in 5 years, 10 years, or even next year." Being stuck in a job, or feeling trapped can lead to feeling of boredom and monotony. That can take the joy out of a relationship.

There could also be some type of sexual abuse or trauma. Maybe something you do or do not know about. If she was abused or something like that, then she may have some issues that make her not want to be intimate. For people that struggle with that, sex can make them remember what happened to them or they may even feel digusted by sex, or feel dirty or shameful about it.

I don't think anyone on this forum can tell you why the passion is gone. You are going to have to have a heart to heart with your significant other and figure out what is wrong. Do you give her suprises, unexpected flowers or gifts. Do you have mutual activities or go out on dates? Do you get along, or is there a lot of hostility and anger? You having sex with other women may be giving her feelings of resentment. There could also be the issue of diseases. She may not trust you to be clean and is fearful of picking up something.

My best advice is to try and start talking and digging down deep to find the issues that are causing distress in your relationship.