Hi, welcome
So, your son has lived at home and has a GF and she came to live under your roof from her parents place. Now they have both left your home. You put it "now my son has left me" yet you didnt include why he left home??
I assume you had a heated discussion about something?
Any family would be concerned and needed reassurance as to why their daughter left home. Yes, her family might not be easily accepting of your family and might even "cling" onto concepts that raise their concern via criticism. This is commonly caused by lack of familiarity, eg parties not knowing each other enough. That can be resolved by a meeting with them to try to find common ground.
As for your son he is a adult now and I would suggest that you contact him anyway you can with a simple message "we are here at all times for you both, when you want we can help. Love you"
Until them allow them to spread their wings and that has been more difficult for you with him being an only child along with you always being close. But times change, circumstances change and he is exploring a new love and life. Not letting him go freely is a restriction he wouldnt want.
I suggest throwing yourself into hobbies, interests, support groups etc to make yourself as busy as you can and wait till he contacts you.
If you'd like to talk further and discuss why he left home I'm here for you.
TonyWK
