I think it's pretty cool and admirable in and of itself that you have such a solid understanding of emotions--or would it be more accurate to say that you attempt to have a solid understanding of them. Pretty much sounds like you already have an idea of how you're going to deal with the situation, but you're starting to feel uncomfortable because your appraoch to this doesn't really mesh with how society (most people) believe that this situation should be dealt with.
Although completely different both sides arent without merit. Were I to be in the same situation I'd probably hold the emotions in and let them hopefully die out slowly. But then again I'd do this in order to avoid awkwardness with the individual who i held affection towards, and perhaps conflict with the partner of said individual. The social consensus is that if you try to get between two peope in a relationship, then you're a home wrecker--which isn't a completely false consensus depending on how you go about portraying your emotions.
Forceful or manipulative persuits aren't okay, but if you’re just telling the person you like them, while outlining your intentions (that you just want them to be aware of your feelings, but don't intend to force their hand or get between their current relationship), then it should be fine. You respect their current situation and you also respect your own emotions by not forcing yourself to prioritise other's over yours. If the person does leave their current relationship for you, then it's their choice (it probably also means that within that relationship things weren't to their liking).
If a third parties confession is seen as the end-all-be-all of relationship armegedon, then obviously the people with such a mindset don't fully themselves respect or understand the foundation of what a 'true' or 'solid' relationship is.
People are quite stingy with their emotions nowadays anyways, and that does more harm than good. All this stoicism and refrain causes some unnecessary pent up sh-t, and indirectly constructs pessimistic outlooks on the outcome of letting your emotions be known. For a supposedly 'civilised' collection of organisms its pretty primative for someone to get to easily offended by another's simple expression of emotion...mostly because you're not taught exactly how to deal with said expression in a 'civil' manner. -_- no, punching someone or verbally abusing them in an attempt to show dominance or possessiveness over your possession (oh, sorry, your 'partner') isn't civil.
As a human you're entitled to an expression of your emotions. As a human the person you're confessing to also has a right to express their own emotions in response to yours. Be that positive or negative, you won't know until you try. Worrying is a waste of imagination, and listening to society is a watse of your emotional commitment. Just tell them and then they can decide what to do from there; if they want to stay in the relationship your confessions probably gonna do very little to shake it, might actually make their bond stronger.