Despite being in a happy long-term serious relationship, I've been in love with a man I met online for the past ten years.
Are you ever going to stop talking to the man online?
No, I want to marry him
If you are in a serious relationship, you should not be talking to a man online. It is a breach of trust and it is betraying the relationship.
Your reasoning is very idealistic and narrow minded. I don't care.
Don't you think  what you are doing is hurtful to your partner though? It's wrong to be in love with multiple men at the same time.