Mamoru & Seiya
In case you are unaware, Usagi was highly sought after by Seiya Kou in the last season, Sailor Stars. Seiya is really Sailor Starfighter from the planet Kinmoku, along with the other two Sailor Starlights and their Princess Kakyuu. The Starlights disguise themselves on Earth by taking up the bodies of males. So, to all of the girls until they learn their true identities, Seiya, Yaten and Taiki are simply guys. There are a ton of Sailormoon fans who just love to ship Usagi/Seiya, despite already knowing how the story ends. Most (but not all) of these shippers are usually people who hate Mamoru for one reason or another. Spats often occur over this. Usagi/Seiya sites spring up often spouting lies which they attempt to pass as truths instead of "what-if"'s, causing some fans to become confused as to what is actually going on. Since the Stars anime has not been commercially released in North America, confusion and misinformation on the issue still runs rampant.
I will go over 6 very basic, easy-to-follow, completely factual explanations to straighten out any confused souls out there.
1. Mamoru loves Usagi; Usagi loves Mamoru. - I don't really think I need to explain this.
2. Seiya falls in love with Usagi. - At first, one could be unsure if Seiya really crushes on Usagi, because he mentions that Usagi just reminds him of Princess Kakyuu. But as time goes on and they get to know each other better, it's pretty blatantly obvious that Seiya falls for her. However, Seiya acknowledges that his love is unrequited.
3. Usagi only likes Seiya as a friend. - Yes, until the very end. Usagi really cares about Seiya, but in the same way that she cares for all of her friends. Seiya tries to express his feelings verbally several times, but Usagi just doesn't seem to get it until the last few episodes, because she just doesn't see Seiya that way. Seiya realizes and eventually accepts that he is in a "one-sided love". So why can't everyone else?
4.Seiya does not hate Mamoru. - Some people seem to claim "Seiya hates Mamoru and tries to steal Usagi from him!" Okay, that's just not true. Seiya happens upon a photograph of a happy Usagi and Mamoru. He frowns at the photograph and sets it face-down, remarking, "Stop making her so sad." Us viewers have to remember that Seiya is seeing the situation from an entirely uninformed view. He doesn't know about their past together as Serenity and Endymion, the deep bond they share, and he certainly doesn't know that Mamoru hasn't been writing because he was killed by Galaxia and never even made it to the US. He just thinks that Mamoru is some guy who went off to the US and dumped Usagi by never contacting her. Seiya is just upset because he thinks Usagi is clinging to someone who no longer wants her.
5. Mamoru does not hate Seiya. - I have no idea where people even get this from. They only meet one time in episode 200, and both are very civil. Seiya tells Mamoru to take good care of Usagi, and he also tells Usagi that he's glad she got her boyfriend back. Mamoru never seems angry or jealous of Seiya. If anything, I would be led to believe that Mamoru is grateful to Seiya for protecting Sailormoon when he couldn't be there.
6. Seiya is really a female; Sailor Starfighter. - Usagi has proven time and again that she is not interested in females. There's a small miniscule detail that everyone loves to forget - Fighter's not really a guy. Sailor Starfighter is 100% female, and only uses the body and persona of Seiya Kou as a disguise on Earth. Haruka flirts with Usagi endlessly, and she never acts like she's interested in that. On the contrary, she usually gets mortified and finds some random excuse to suddenly run off. Although, I believe that Haruka really only does it because she knows it makes Usagi uncomfortable.
Just a few more little tidbits before I close:
Seiya is a better match for Usagi. - Okay, fair enough, they are more alike than Usagi and Mamoru. However, as they say, opposites attract. Usagi and Mamoru may be different, but they learn and grow from each other because of their differences. Mamoru teaches Usagi the wonders of being calm, and the importance of school studies, and responsibilty. Usagi teaches Mamoru how to open up and be himself more, and most importantly teaches him how to love. Mamoru's more calm, collected (though not always) persona balances out Usagi's over-the-top, loud persona. I agree that Seiya and Usagi look cute together, but it's just not meant to be.
Their relationship should have happened. - Hmm. Sometimes stories just aren't the way you think they should be, and often it's better that way. Would Romeo and Juliet have half the impact it has if Romeo and Juliet didn't take their lives at the end? I can guarantee you no. Seiya's one-sided love for Usagi adds meat and drama to the story, but that doesn't mean that that's how it's supposed to end.