Okay, this is a thing I've heard ALL around the internet...let's get one thing clear. Seiya NEVER did try to steal Usagi from Mamoru...there's more below.
People think Seiya tried to steal Usagi while nested on Earth, that's not true. Seiya flirted with Usagi, sure yeah but that happens to people right? Seiya got close to Usagi and thought that Mamoru was deciving Usagi while in America. Not getting any calls or mails, Seiya assumed he was cheating on her or just ignoring Usagi.
In 181, Seiya asks while thy're on the Ferris Wheel when Usagi admits she hasn't gotten any replies from Mamoru Seiya asks "He sounds like a heartless guy, is he deciving you?" Usagi replies "That isn't true. Mamo-chan is always thinking about me." Seiya apolgizes when seeing Usagi a little upset from the question and doesn't bring up the subject for the rest of their date. Okay, they went on a date too but Seiya wanted just to spend time with Usagi it's not like they kissed on the lips or anything. When the power went out, Seiya held Usagi close and Usagi felt Seiya's warmness and just compaired it to Mamoru's seeing that it was different.
In 184, there is more edvience that Seiya thinks that Usagi is being decived. Whie at her house, Usagi is downstairs or whatever making the tea and Seiya see's a picture of Mamoru and Usagi. Turning the picture down flat on the desk Seiya says "Don't make her so sad."
In 180, there is a bit more edvience Seiya thinks Mamoru is not in love
When the conductor from the concert asks "Seiya, is she your girlfriend?" referring to Usagi, Seiya replies "Something like that." and Usagi says "Just a friend!" and Seiya replies "Isn’t that kind of cold?" This shows Seiya was maybe getting more of the idea that Mamoru was choosing not to answer Usagi.
I think people get the idea of Seiya "stealing" Mamoru away from 176 when Seiya asks "Dumpling, your boyfriend is overseas right?" and she replies "Yes." and Seiya says "I have a chance." Usagi is confused, but Seiya was gaining enough knowledge to know Usagi was probably being ignored.
In 194, Seiya’s packing up the school bag in the classroom and thinks "this is my last moment as Seiya Kou." Seiya remembers some memories of the school and thinks "It was fun." Passing by Usagi’s desk, Seiya takes a rose and places it over a drawing Usagi drew of her and Mamoru. Before the rose is placed on the desk, Seiya sees Rei open the classroom door asking if she (yes, the Starlights are girls) seen Usagi?" Seiya takes the roof when Usagi’s friends takes the gym. On the roof, Sailor Moon is fighting Nyanko when Seiya is dressed up as Tuxedo Mask and throws a rose. Sailor Moon is stunned to see Seiya and the light says "If you hurt that girl, I’m not going to forgive you." Nyanko says "That’s funny! What can you do?" Seiya transforms into Fighter and she and Sailor Moon take down Nyanko. This is when Usagi has a break down on the roof yelling she wants to see Mamoru. Seiya asks "Am I not good enough?" Seiya is asking this because the Starlight thinks Usagi is being ignored.
The episode later, 195 after Seiya’s feelings and before the kiss Seiya says "I hope you get back with your boyfriend soon." Even in the final battles with Fighter aka Seiya protects Sailor Moon (Usagi) she’s doing it for her love Seiya stated it is a one-sided love but that doesn’t mean Fighter (Seiya) can’t flirt with her, the two have a playful relationship.
In 197, after Sailor Moon discovers Mamoru died she starts crying and Fighter can see how upset she is. Now, would someone try to steal a person from someone else if they were THAT sad? No, of course not! Nobody but Galaxia knew Mamoru was dead...Seiya only made advances on Usagi because the light thought Mamoru was ignoring Usagi.
In 200, the final episode while on the roof top Seiya says to Usagi "I’m happy you got your boyfriend back." Usagi says "It’s because you were with me, Seiya. It’s because of you I could hang in there." After this Seiya says "I’ll never forget you." Usagi says they’ll be friends then everyone is poking fun at Usagi for not understanding. When their princess says "Now, let’s go." Seiya stops and says "Mamoru-san. From here on out you need to protect her." and Mamoru is silent for a second and Seiya said "Just kidding! That’s a line from some cocky bastard." Mamoru knows Seiya is saying just protect her and replies "I understand." Seiya says the final good-byes and they Starlights leave to go home.
So, Seiya NEVER NEVER NEVER tried to steal Usagi from Mamoru.
