That's just how it goes, she assumes you're a fake natty because most guys are tiny and you never see guys that big in the game so she thinks you're on steroids. Sarah is a noble natty lifter.
gotcha. Kinda sucks, but hey maybe a small quest line or something can be made to prove you're a natty, or like a check system exists where if you reach a certain point and less than x amount of your muscle xp is the roids you can convince her.
On another note: how does one increase agility and such? Cardio and strength, super easy. The other two, not so much. I can't seem to find any exercise that touches them and fights don't help (granted at like muscle level 4 everything dies in like 1 to 2 hits).