eating a bechor which one made a ba'al mum

Rashi (Devarim 14:3) writes that the issur of eating a to'evah prohibits 1) shechting and eating a bechor which a Jew has caused a blemish to, as well as 2) eating basar b'chalav. 

Tosfos (Chulin 115) asks: Why does the issur of eating a bechor which was intentionally blemished apply only when a yisrael causes the blemish?  Just like basar b'chalav cannot be eaten even if cooked by an aku"m, so too, even if the blemish to the bechor was caused by an aku"m, shouldn't it be called as toevah?   

Tosfos answers that we know a blemished animal is not a toevah because animals which are blemished pesulei hamukdashim are allowed to be eaten.  Therefore, the pasuk cannot prohibit eating any bechor which has a mum -- there has to be a case which is permitted. 

Tosfos seems to straddle the fence -- M'mah nafshach: If a mum is a toevah, then every case of mum should be assur.  If mum is not a toevah, as we see from psulei hamukdashim, then even if a yisrael causes the mum, the animal should be able to be eaten.  How can you split the baby? 

Maharal in Gur Arye  rejects Tos.' answer and offers in its place a bit of lomdus.  He explains that it is the metziyus of basar b'chalav which is prohibited, irrespective of who does the cooking.  You can't say the say the same thing about a mum.  The proof is simple: Were mum a toevah b'metziyus, then even an animal born with a mum should have issurim attached it it.  That's obviously not the case -- an animal born with a mum simply lacks kedushas bechorah; there is no status attached to it.    It's not the mum itself which is the problem, but rather it is the ma'aseh aveira of creating the mum which the Torah prohibits.  Therefore, the din of toevah applies only where a yisrael does a ma'aseh aveirah and causes the mum, but not where the mum is produced by an aku"m.