We're speaking about two different things in a medieval society. Love and political marriage. They can overlap but they are two distinct things. The northern lords wouldn't be phased by any amorous feelings Jon and Dany have for each other. They don't know Dany and for all they know Jon is just smitten and in lust rather than love. And remember that love is what got Robb and a lot of northerners slaughtered by the Freys. The Northern Lords would however care about a political marriage and whether love is involved would be a distant comcern. And tho they know what's at risk they will always prefer Jon marry a Northern lady and not the daughter of the Mad King. Tyrion is likely thinking about the risk of the love. Not the political marriage. Them having a falling out because of a failed trist could literally mean the death of every man, woman, and child on the westerosi continent. Everything is riding on Jon and Dany cooperating with each other on friendly terms. And there is the possibility that an amorous relationship could be a weakness and compromise Dany. A person like Tyrion will be wondering if Dany would be willing to kill or let Jon die if she's in love with him. Even before Tyrion knew about the White Walkers and the Army of the Dead he believed that Westeros' future depended on Dany. He went so far as to risk the entire war because he was unwilling to encourage Dany to fly her dragon's into battle. Hell, he even told Dany to not rescue Jon which would have resulted in no Wight to show Cersei, Dany wouldn't have seen the Army of the Dead, Jon would be dead, the North would think Dany betrayed or outright murdered Jon, and there would have been no coalition to face the Army of the Dead. Tyrion was willing to risk all that to keep Dany out of harms way. IDK what sort of plans Tyrion might have for himself, but he seems to really be all in for Dany and wants her on the Iron Throne. Love with Jon is a potential threat. A potential wrench in the gears. Tyrion must recognize this and is probably just concerned.