You feel the feelings, consider them and make decisions. There's no sure fire answer to the question.
I have lived with my husband for 14 yrs and my boyfrie d has lived with us for 10 of those years. I suspect we will live under the same roof until we accomplish building multiple homes on one property so we can each have our own and whoever else is in the mix can too.
We are definitely life-partners and wedding ceremonies are arbitrary.
"Love As Thou Wilt"