Let's start with a little historical perspective. Rompers are not new, and they are not feminine. The romper's 18th century ancestor, the skeleton suit, was exclusively for small boys who were too young for adult clothing and too old for baby dresses. Yes, dresses. Calm down.
The modern romper (sometimes called a creeper) was introduced in the late 19th century as a casual garment for toddlers -- male and female. It remained popular through the 1960s, by which time it was also worn by older children and adults as a leisure style. In other words, for a couple of generations it was unisex, though there were "feminine" and "masculine" variations for folks who wanted a more gendered version.
When gendered clothing became the norm in the 1980s, rompers apparently got sorted into the "feminine " category, according to the same cockamamie logic that made kitties and purple feminine. There is no natural law that determines this; it is a cultural mystery, and it is both arbitrary and negotiable.
So if a guy wants to wear a romper, by any name, it should be no big deal. Simmer down, Internet.