He was afraid of her ending alone not that she might be a lesbian, Willow sister is a big time lesbian and till now Gunner has not show any negative opinion on the matter
Didn't say her ending up alone wasn't a reason but it's not the only reason.
The previous conversations between Gunner and MC where he says she has no Interst in Men.
The MC himeslf took that as possibly Gunner implying she might be lesbian.
And as gunner then go's on to point out her sister is a lesbian.
There was an unspoken implication there that he thought a possible reason for her lack of interest in men.
Could be that Willow might be like her Sister.
Seems it's just the MC that figgered out she was just a Tomboy.
And Gunner may not be against it but he do's point out it in his own way.
That without willow hooking up with a the MC it lessens his chance of a great grandson.