6 .
Why ~ if it's widely believed to be so enjoyable ~ is adultery* a sin, in the eyes of the Church as a whole and Catholics in particular?
(* = the pursuit, and having, of sexual relations with anyone to whom you are not married: i.e., before or outside a marriage)
(Opt for this Answer if you believe all the others are more or less equally important spiritually, and inextricable)
If the active adulterer truly respects the person they're 'after', they shouldn't wish to impose their own selfish desires ~ while also enmeshing that person in complicit sin
Sex between people who aren't married to each other could result in the birth of a child (whose parents could then not offer the 'model couple' upbringing context) ~ unless either or each were using contraception, which in turn would be defined as sinfully trying to prevent or subvert God's will to bring a child into the world
Adultery has been forbidden since the Ten Commandments, and most people in their right mind (out of the heat of the moment, so to put it) recognise it usually creates more problems than it might appear to solve