People were saying that it helps to know a person better, warts and all, before you make a full legal commitment.
By living together you're already making a pretty serious emotional commitment, and if that commitment is as serious as you both think it is before biting the bullet and choosing to move in together, it should survive that experience well enough to make you both want to move to the next step and make that commitment full and permanent.
You test-drive a car because you are perfectly free to buy, sell, trade, redesign, or disassemble and reassemble cars or even leave them in the junkyard. A car is a commodity.
A person, though, is emotionally, spiritually, and physically designed to do one of two things when they have sexual relations:
#1 is what you do in marriage, because marriage (I'm talking about real marriage, which includes a sacred public vow of lifelong loyalty) is a secure setting for total, unreserved, heart-wide-open self-giving.
#2 is what you do in living-together. Even if it's living-together-with-option-to-marry. People WILL close up emotionally. They WILL train themselves to withhold ultimate trust. They WILL shield themselves from real openness, because there's a BIG possibility of a break-up, and emotional pain hurts like hell.
And people who do Option #2, living-together, can find it very difficult to transition inwardly into Option #1. They're not learning peace and trust. They're learning performance-anxiety, audition-anxiety, probation.
It's an inward thing. It affects the soul. I think there's a part of you that already knows this.