When you take a vaccine you inject nasty organisms (mostly disabled to put it simply) in your body. Do you want to become sick? No.
The use of desire and other emotions, feelings and experiences in the path aims to free oneself from samsara, not binding oneself to them. This makes the difference. This and the proper method, guidance and so on and so forth.
It's a fight fire with fire situation if you will.
For instance, one may get laid to have fun, taking delight in samsara. Or one may practice karmamudra to swiftly attain enlightenment. One may also screw oneself completely if one decides to do this without proper guidance and preparation.
See it this way if you want: according to Vajrayana enlightenment is already present, from the very beginning, but we fail to realize it. An ashtray can be made of gold and a Buddha statue can be made of gold. You realize gold is the same in both. They have the same nature: gold and the gold of the ashtray isn't less pure than the gold of the statue. However, having this pure vision is easier said than done, but this is fundamental to Vajrayana practice.
Then there are all those yogic methods that deal with our body and especially our subtle energies. By controlling prana your mind becomes easier to tame and one can have better access to meditative stages important for the progress of the practitioner. So this very body is used as a base to the practice and there's a sort of loop regarding the way one works with energy and mind, both fueling each other. In the end one is not doing this for its own sake, but aiming the realization of enlightenment.
So, the fact that one uses this set of tools (emotions, body, subtle energies and so on) doesn't mean one wants to stay under a samsaric experience. Renunciation must be even stronger and deeper as one may have the temptation to go astray when dealing with these methods. They are not without danger, so a strong, well rooted and clear renunciation must be the foundation of all this process.