not really no. marcos struggle would be the same, but it would make more sense if star was a boy. hes so resistant to admitting he has feelings too and keeps going out with girls hes not interested in.
if marco was struggling with being gay for star, then being so unattached to literally all the girls in his life would make sense. remember that he was gushing over star and mewni while dating jackie so bad that she left him. and he doesnt care about janna, nor should he but still. he had no interest in starfan, only went to kelly on the rebound, and only hung with hekapoo for fun.
i think you shouldnt think in terms of gay or bi. thats somewhat limiting. star in this instance would be a solid 100% fag, but marco likes what he likes. not bi, not anything else. lets say hes gay for star only, but hes also in love with star so no girls are welcome in his life