The colonization of America occurred in the 16th century, when boat trips were very hard, only sailors, soldiers and strong laborers could endure such adventures. There was no place for white women.
In later centuries when there were passenger ships the mixture was already a fact, and civilized mestizas and harnizas could be just as desirable as a European and there was no problem, it was all a matter of caste.
Northern Europeans chose the most easy and natural option, which is to transport women or couples to their settlements on passenger ships.
In short, mixture was forced, there was no choice.