Marriage!

[Lalaith]

Teleri men wouldn't necessarily be weak, they just had different priorities to the Noldor.
There was a mortal woman who had a love affair with an elf, she was called Andreth and he was Aegnor (Galadriel's brother I think?) But it didn't work out.

And in defence of Sam and Rosie, a family of thirteen children was not particularly excessive in Georgian and Victorian England. (Look at how many children the Old Took had, btw)
Most couples had a lot of babies (no contraception!) but often not all survived the childhood illnesses and post-natal complications that abounded in the days before innoculations, antibiotics and so on.
Some families, however, due to perhaps a combination of genetic luck, better access to healthcare and good parenting, did manage to bring all or most of their children to adulthood. The Gamgees were clearly in this category!