If God's institution of marriage is for 1 person for life, then I'd
have to say we're created monogamous.
Sin nature however might play a huge roll in the corruption
of that ideal. & depending on how much we feed our sin
nature, it can have alot more power over us than most
other people who don't allow themselves to be tempted
by others or other things.
In other words, a person who continues in porn will only
feed lust. We shouldn't wonder why they're having so
much trouble with sexual immoralities.
Galations 5 tells us how to avoid living defeated in sin, by
walking in the Spirit of God. The closer we are to Him, the
less likely we'll be to fall into traps & snares.
I think there's alot to mid life crisis issues - I've always said
that adultery often has more to do w/ the person committing it
than their spouse; they have internal issues that don't
necessarily have anything to do w/ lack of love of their
partner. (wrong as it may be)
I'm not entirely sure what you mean by this,
our common sense shouldn't be violating scripture principles.