There are multiple vectors for this without ever doing anything with regards to a "Love Potion" such as stop making their life comfortable, increase the death rate, make them poor, reduce their education, make the environment hotter. All of these things will cause people to reproduce more.
As far as an actual love potion. Basically, you just need to find the chemical concoction that is produced when people are horny... those would be interesting experiments to be a part of... and then just create a pill or gas of the concentrated concoction, or one that triggers the release of that in the body naturally.
You could also create a nanite swarm that hijacks the body of a person to make them horny/reproduce. There is no reason that this isn't possible. We're just technologically not quite there yet.
As far as "safe sex". No such thing really. You have enough sex and you'll win the lottery or if your always horny or whatever you'll forget at some point and this would happen more and more as the situation is prolonged because people would rather screw unsafely than go out to a store and kill the mood especially if you get them horny enough...
Of course the issue with anything like this is that it is a form of rape and any political body that authorizes it will be lynched. If I were in this predicament I'd go with a physical military assault than rape by proxy of an entire colony.