I have questions about when sex became taboo for children to learn about. Look at historical dwellings and family sizes and try to imagine how they kept kids from figuring it out from firsthand observation.
Typical pattern in Western Europe was women married at 19-22 in the 1500s. You can find outliers like Tuscany in the 1400s where it looks like that number is 17-21.
Typical ancient Roman marriage patterns were for women to wed at 20-25, it seems.
There's outliers, of course. Especially with elites: where political and other concerns often created pressures for early marriage.
Looking at the outliers in today's world, you can find e.g. in Sub-Saharan Africa about 35-40% of women wedding before age 18. So there's really only tiny pockets of the world in space and time where it seems that the average marriage would be under age 18.
So the answer is, it was always taboo for kids or anyone else to watch. People may not have always been able to hide it effectively, but they were always at least trying.
Not sure why you think this. Plenty of other animals become violent during marrying seasons, it's often an explicit part of their sexual behavior, at least for males. For example, most horned mammals (deer, buffalos) have violent conquests in the mating season.