I'm also not the biggest fan of strict mutual exclusivity - whether it be in research or some other aspect of the game. I don't hate it, but I think a 'soft' form of exclusiveness works better: it allows people to mold their strategies more to their own personal preferences while still preventing players from getting the best of all words.
To give an example, let's say there are two techs, "laser accuracy" and "laser power." With regular mutual exclusiveness, researching one prevents you from researching the other - you have a binary choice between accuracy and power. With a soft form of mutual exclusiveness, there are several ways of going about it. One method is to make researching the second of the two take significantly longer or cost much more than it would (or a combination of the two). Another method is cause each tech to have reduced effectiveness if you research both (or, when actually using these techs, choose whether to have a very accurate but weak laser, a very strong but wildly inaccurate laser, or an all-around mediocre laser) - how this would be be done depends on the nature of the tech and the game.
In the vast majority of circumstances I think some form of soft mutual exclusivity is more fun than straight up "if you researched this then you can't under any circumstances, no matter how situations have changed, and no matter how much you are willing to put into it, research that." Sometimes that is the way to go, though. In my example of laser research, I'd say no. If there's something that fundamentally changes some part of an aspect of your nation, maybe. Things like "free the slaves" vs. "enslave everyone!" don't seem fundamental to me if they're including as research. If position regarding slavery is determined by faction choice, on the other hand, that's an entirely different matter. But to me, if I can research it, then I can choose whether or not to actually use it, regardless of whatever other technologies or policies I have at my disposal. However, I'm all for not being able to use certain techs/policies in conjunction with others - or getting severe penalties to their effectiveness if you do.