In the Lore I haven't seen the Septims ever getting married to other so called great nobles.There aren't a lot in Tamriel.The Septim Dynasty lasted 433 years(17 generations).It just doesn't strike me that they would marry "whoever they wanted", I mean royalty with poverty doesn't mix in most fantasy worlds.What if the Septims are inbred.Like the Targaryens in ASOIF novels.I mean, a couple of them were insane(Potema,Pelagius Septim III), probably as a result of not having the proper genes.Anywho, opinions and discuss.

Tiber was the first Septim and when he founded the Septim Empire, the 3rd Era began.

I KNOW THAT.Never mind I looked it up.433 years.

We know at least one Septim married to an other family (the Dunmer Empress Katariah). Nothing really suggests they are inbred. We just haven't had a lot of background information.

We don't have evidence to support the contrary.