Jedi are highly capable and very attuned to the Force but that comes at the cost of making them vulnerable to corruption and falling to the dark side. Romantic attachments would increase this risk significantly. Especially for Jedi in training it would be highly dangerous to pursue a passionate relationship.
Note that not all force sensitive become Jedi, and some Jedi retire from the order and stop being Jedi, sometimes exactly because they wanted to pursue a relationship.
From this it can be assumed if you stop actively using the force, the risk of you falling to the dark side significantly decrease and so the jedi order does not mind.
This means having family lines that are strong in the force probably wasn't that uncommon before the Empire era. You'd just have the occasional family line with a bit more force sensitivity in it.
There probably weren't that many breeding programs for the same reason we don't have that many in reality. It'd be creepy and difficult to organize. And any significant changes in force strength might take a very long time to materialize. Most you would have would probably be two families arranging a marriage between them.
Then with the Empire era I'd assume Palpatine would genocide not just jedi but any force sensitive he could find.
And that's if force sensitivity really is inheritable. It could be more of a fate thing. The skywalker line is fated to be strong in the force. Then it'd be much more random. Even then Palpatine might still have genocided family lines just to be on the safe side.
Also, the jedi order in the era before the Empire was very strict but in some of the Expanded Universe material it's indicated Luke Skywalker's reformed jedi order were less ascetic and allowed for couples (probably would be a good idea to still forbid it for jedi in training).
As a side note, when the only movie that existed was A New Hope there existed a fan theory that Obi-Wan was a clone (because clone wars was a line in the movie), and that his real name was OB-1 (serial number) and that Jedi were originally manufactured clones.