Neither of these states, celibate or monogamous, is necessarily perceived as "natural" for a man. I think most men can be aroused by a large number of women, and if aroused, could have intercourse almost any time: that is, if he were to do what seems to come "naturally."
Some men-- acting at a level above their instincts and appetites -- have a capacity for life-long celibacy, as Jesus and Paul both said, and all the rest for life-long monogamy. The Western Church chooses priests from the ranks of the celibate.
Nobody is "forcing" anything. USCCB guidelines require approximately 8 years of education, training and discernment to become a priest: four years of undergraduate study of philosophy and four years of graduate study in theology (for men coming directly from high school). During those whole 8 years they are getting a very good idea of whether they're cut out for lifelong celibacy, or not. You'd maybe agree that if a man could be more or less serenely celibate between the ages of 18 and 26, he probably has a vocation to celibacy.
Lots of young men leave seminary without being ordained, because they experience a strong desire for marriage and family. The percentage of seminarians who drop out without being ordained varies (different places, different years) but I've read that on a rough overall average about 50% of seminarians do not go on to become priests.
So men who are celibate are given every possible opportunity over a number of years to discern whether they have a vocation to lifelong celibacy.
If married men feel they have a call to Holy Orders, they can become deacons, which is an ordained clerical vocation just like the priesthood is.
There's no force or coercion here at all. If anything, the lengthy years of preparation --- during which they can leave at any time --- tend to thin the ranks down to the ones who really have a celibate vocation.
You can train people for 50 years and it won’t change the fact that human beings are programmed biologically to want to have sex. This is indisputable.
There is nothing religious about celibacy.
For its first 300 years priests married and had children. Popes begat Popes.
God made man and woman to desire sex. When you create a situation where you cannot have it, you will have problems.
Until the Catholic Church reverses this rule, the numbers of Catholic priests will continue to shrink at its current alarming rate.