Can lymph nodes in your groin be swollen for 6 months and painless for any other reason than something serious?

Yes: There are many potential reasons for this, but only a doctor examining these and evaluating the potential causes can work this out.

Monitoring Centers: Lymphatic system absorbs/carries things too big to go into the arteries/veins like bacteria eaten by white cells/digested food. Lymph nodes "taste" the lymph everywhere for bad stuff like bacteria/viruses/cancers, etc. When it detects a problem, it alerts the immune system and your body reacts. They enlarge in response to the inflammation they create when they react. They shrink when all is well.