Of course it's completely fine in your opinion... but it's fundamentally not, because You started off that point with "in my opinion...". I'm fairly sure you're more insistent than I am that other people's opinions ought to be respected.
We had a debate about that the other day around whether people's opinions about medical treatment ought to be respected even if they were based on no evidence. That time, I felt that they ought not to be but you were the opposite. Possibly the difference is that you could legitimately argue in some cases that it was more personal in that case, but sometimes medical decisions do have a knock-on effect on society. But I digress.
As far as I can see there are no objections to gay marriage that will stand the test of time. That's an opinion, though, and we ought to wait and see. I don't think you have any factual basis, yet, to argue that all opposing views are invalid.