I agree...seems it would be selfish to deny your life partner/spouse in order to avoid pregnancy. That would lead one to seek out affection in another therefore sinning. We simply cannot afford to support and raise the big families of yesterday.
In practice, NFP couples testify that periodic abstinence typically leads to greater unselfishness. The spouses devote theselves to finding different ways of expressing love other than intercourse, during those few (5-7) days per month when they are abstaining to avoid pregnancy.
It can become a "courtship" period within marriage: a little writing an intimate poem about desire and longing, a little watching a movie and retiring perhaps to separate beds, a little romance and smooching, a little deliberate thought about "What will really show him how much I love him?" "What would really make her smile?"
And, naturally, a lot of special excitement --- spurred and heightened by all that anticipation --- when they come together again.
NFP has its stresses, and it had its rewards. I've often heard couples say it adds a real depth of meaning to their sexual relations. I've never heard anybody say it made sex seem shallow and boring, which is the case with so many couples who do not practice any sort of abstinence. And boredom with sex --- dwindling of pleasure and loss of meaning --- is what leads to the death of many a contraceptive marriage.