I guess all women in the world are being "pounded by bigger, bulkier men"? You do realize that individual women have different tastes in men and this exact chapter explained, VERY CLEARLY, that Runa's type is a "genuine person who loves her for who she is". Could that be a "bigger, bulkier man", absolutely, but right now it's the MC because he loves her for who she is.
Does MC have self-confidence issues? Absolutely! Does that mean Runa would cheat on him? No. It doesnt. Plenty of men who lack confidence are in loving relationships with partners who support them. There are also plenty of "bigger, bulkier men" that are cheated on because their partners are pieces of shit. It's called real life.