It's very hard to think of a sin which doesn't have that dual role of being, in part, both the effect of previous sin and the cause of further sin.
"To say that contraception causes moral failure denies the empowering work of the Holy Spirit to give people the ability to resist sin."
Not so: that does not follow. Nobody said that sin is an irresistable or inevitable cause of further sin. We all agree with the "empowering work of the Holy Spirit to give people the ability to resist sin."
Don't we pray for that every day?
Not all of us pray for that, or even think about sin on a daily basis.
When I pray, it’s not to ask God to help me NOT do something, but to thank Him for what I have.