Disclaimer: I do not in any way own Hetalia
Belarus had never harboured any romantic love for Russia. Of course she hadn't - he was her brother!
But that is not to say that she did not want to marry him. Not for (romantic) love, of course not - but for more than a few other reasons.
One. Belarus knew exactly how easily Russia could get hurt. If they were married, though - if it was Belarus, she could make sure he was never hurt again.
Two. If they were married, then the two of them would be together - and that was halfway to what Belarus had so desperately wanted for the longest time, which was for her family to be united again.
It was hardly the poor girls fault that no one had ever really taught her to express such things. After all, marriage did mean something different to nations - but if one brings it up, everyone tends to assume that it was meant in a human way, in a romantic way, which had never been what Belarus meant at all!
Sorry I haven't posted in a while, here's this thing I wrote in a fit of rage over how people treat Belarus