Put yourself in Usagi's and Mamoru's shoes. You're an 8th grader and every day you run into this ONE guys who always picks on you and pisses you off to NO end and you can't stand him (even though he's cute). Unless there's a special reason, you're not gonna start falling in love with him.
Now Mamochan, he's in college in the anime, and he always runs into this whiny, noisy 14 year old who always annoys him. Mamoru is the guy who's grown up all alone and has built up defenses around him to prevent himself from being hurt (he lost his loved ones in a car crash when he was little). To him, trying to get close to Usagi, letting alone falling in love with her, is simply OUT of the question. What self-respecting 17/18 year old would chase after a 14 year old?
Then one day *wham* you both remember in a past life you guys were in love. Suddenly, the person you were so annoyed by shows you a different side to themselves. If you were faced with the memories of someone you loved DEARLY in a past life, and the person is standing before you right now, what would you do?
The next rational step is to try and get to know each other in THIS life and see if that love is still there right? So that's what Mamo and Usa do. (once he's finished being brainwashed and gets his memories back) you can see they spend time together and that one scene at the beginning of SMR when Rini/Chibi Usa comes, UsaMamo are in a boat and when she gets off onto the dock he tells her 'you're such a klutz, but you're MY klutz'
Now if he only loved Usagi for what she was as Serenity then why would he be with her now in the present? She's nothing like the old Princess. She's totally different!!! Yet Mamo got to know the real Usagi, not the princess form and loved the real Usagi herSELF.
So whether or not they loved each other BEFORE they got their memories back is irrelevant. What's important is the fact that they *DO* love each other for themSELVES in the present. And that is obvious in SO many scenes
Also important to remember, this is what happened in the ANIME. In the MANGA, however, Usagi and Mamoru fall in love with each other for WAY before they learned of their past lives, and the manga is the way it was really supposed to be. If the animators had been faithful to the manga, we wouldn't be having this argument because then everyone would have seen how thy fell in love at the beginning ;)
So tell THAT to the people who accuse Mamo of not deserving Usa cause he doesn't really love her in the beginning - *WHY* should he? Why should they love each other at all? They haven't even given each other a chance in the anime to try and get to know each other. All they know is to get on each other's nerves.
The point is, once they learn of their past lives, that is when they decide to give each other a chance. To see what the present holds for the love they once had. And as it happens, both fall in love all over again - but *THIS* time - they love each other for *each other* not for their past lives.