To the son in the middle of his mother and spouse or girlfriend, here's a piece of advice for you—love them equally. While it is stated in Genesis 2:24 "Therefore a man shall leave his father and his mother and hold fast to his wife, and they shall become one flesh," sons must never diminish their love and respect for their mothers.
A married son must always strive to foster an ideal and harmonious relationship between his wife and mother to ensure that he stays grounded and able to maintain that balance. While doing so, a son must always strive to assure his wife that as they become one in marriage, he must live by the principles of honoring her by putting her welfare first. When wives are assured of their husband's love and respect for them, then they will feel protected and secured and will never question any action especially if it concerns their parents-in-law.
A son/husband must always make sure that he does not get to a point where he needs to choose between the two people who are considered to be the most important women in his life. As much as possible, he should be very careful in making decisions that will cause any disagreement in the family. For example, if there is a family gathering, if the schedule causes a conflict with a family activity, then the husband must be able to solve this conflict and weigh which should be prioritized and which can be put on hold or rescheduled. In case this situation happens again, he must now consider giving in to the party who sacrificed first. In this way, both women will feel that the son/husband is trying his very best to please them both.
If in any case the worst case happens, that is if the mother and the wife are not on good terms, then the son/husband must do every possible thing to resolve the conflict. He must stay neutral and not side with any party and make both women feel valued and respected. In this case, I also urge the son to encourage his wife to always maintain her respect for her husband's parents as this is always the right thing to do.
And for the mothers like me out there, here is an unattributed post I read online:
I'm still your mom
When you no longer listen or care what I say, I'm still your Mom.
When you decide I'm old-fashioned and want to go your own way, I'm still your mom.
When you talk back, complain, and argue, I'm still your mom.
When you figure you know more than I do, I'm still your mom.
When you move away and take a piece of my heart with you, I'm still your mom.
And I will love you still as much as the first day I held you in my arms, I'm still your mom.
And I will pray for you and make sure your wings are strong enough to soar, I'm still your mom.
I'll want what's best and will sacrifice my plans for yours, I'm still your mom.
I'll always have room for you and a hug to welcome you, I'm still your mom.
And until my last breath, I'll keep carrying your love with me.
And I will thank God every day for the privilege and joy it's been to be called - your mom.
To all the married sons out there, there is no joy equal to the feeling of being surrounded by the love and service of your mothers and wives at the same time. This is only the situation when being "caught in the middle" is indeed a good place to be in.