How does a having a libra ascendant affect saturn in aries?

It doesn't. Not unless there is something else going on that link the Ascendant with Saturn. Like Saturn being in an opposition aspect (within 8 degrees of an EXACT opposition) to the Ascendant.

You cannot use the signs to find if the two are opposite each other.

It doesn't unless it's in an opposition (180 degrees) aspect. The Ascendant is how you come across to people when you first meet them, your social mask. Saturn is structure, order, discipline, limitation.

your sun might be in your 7th house so that would mean you feel incomplete without someone else and you live to please people and relationships are your priority