@breadbin123
How did you know they were the one?
I don't get this. There isn't "the one", there is "a one".
I can't say with absolute certainty that I am going to spend the rest of my life with my partner, even though I love her now. I sincerely hope that I do but something might go wrong, we might just grow apart.
Now, if we were to break up, after a while we would just date other people (or be lonely cat ladies for the rest of our lives, that's always a possible scenario).
We didn't meet online so it is different. However, when we were starting out as a couple and met up with each other, we liked one another and enjoyed one another's company and were attracted to one another. So we kept going out and meeting up and spending time together. And I didn't go out with anyone during that time when I was getting to know her because... it just never really occurred to me that that could be considered anything but two timing.
Before I met her, I went out with someone else. Only lasted a couple of months because, while they were a perfectly nice person, we had very different interests and just never really clicked. Now, I spent most of those two months, not unhappy, but not sure that I really wanted a romantic relationship with this person. Gave it a shot, parted amicably. And I didn't go out with anyone else during that time because it would have struck me as very disrespectful towards a nice person who was spending time with me in the hope of a possible relationship.