I have found a lot of "rushed" weddings and illegitimate births in my tree. We complain these days about too much sex education for youngsters these days, and foisting birth control and abortions on them, but back in the 1800s, there was no birth control to speak of, and only back street illegal abortions, so if a couple misbehaved they invariably got caught out. It makes you wonder about sex education, though, there was virtually none back then, and a lot of pregnancies must have happened through ignorance.
My great great grandmother was christened the same day in 1821, in the same Church, as her parents got married. I assume he was the father, one of her forenames was his surname, but I shall never know for sure.
It also happened in the early 1900s. My Father's unmarried aunt went to stay with her sister in London for several months, causing some family speculation. Now on Free BMD, I have found the birth in 1916, but don't know how much further I can take it, as child believed to be adopted.
I was interested in the answer re young parents. At one time British girls could get married at 12, and I think it was 14 for boys.