I’m confused about Penny’s wedding

I’m on a rewatch with my gf who is seeing it for the first time. We just finished season 2 and I got a bit confused with Desmond storyline.

Widmore tells him to leave, as Penny already moved on, soon to marry. Which I didnt take seriously but when Desmond is training, Penny drives by and Desmond brings that up. She says they havent set a date yet. Then in the island, Desmond finds a letter from Penny where she says she will always love him and wait for him? Which was written…before he went to prison or when he was in prison. So after he never answered, she moved on but she still loves Desmond? Like what? My gf thought Penny is an asshole for stalling the other guy while still loving Demond and I was just thinking that there must be something we misunderstood.

Can someone clearly explain to me this?