According to my limited knowledge in relation to happiness hormones, Oxytocin is only released when we trust a person, not matter the kind of the relationship (if we exclude the case of motherhood). I tried to explore the 4 hormones responsible for our wellbeing here:

https://medium.com/illumination/did-you-ever-wonder-about-the-function-of-your-hormones-1af5b6b93c47

When it comes to romantic relationships specifically, I believe the release of Dopomine or Oxytocin is exclusive:

If we are still diving into life with a huge psychological baggage and an unfriendly subconscious program; that is we are not yet loving ourselves, the attraction we feel to a person is the by-product of Dopamine, and what we feel in the Mid/long-term is called attachment. That's why people sometimes stay in abusive relationships for decades. Dopamine is addictive and that's its dangerous side effect.

If, on the other hand, we have already worked on our limiting beliefs and re-written our invasive program (moving from the insecure attachment style to the secure one and destroying our distorted paradigms about the world where all the subtle manipulation happens, and moved back to our original center "the principles") so that we can love ourselves, we won’t be attracted to anyone from the first sight anymore. We will need much more than a physical appearance to be drawn to a person. A soul and mind connection is a MUST. We know exactly why we want this particular individual to be in our sacred world.

Only then, Dopamine is replaced by the Oxytocin! I went through this here, should you be interested:

https://medium.com/illumination/shall-we-review-our-love-vocabulary-5f4e5cd9e09