Can long-term use of singulair and levocetirizine (for chronic allergies) cause side effects: liver damage, low sex drive/libido? can't use sprays.

Not really: but you need to keep the follow up with your doctor, both drugs aren't known for the side effects you noted, Singular may cause liver problems, but not related to long term use rather due to a reaction by some patients even after few doses, rare. If you have chronic allergies,you need an allergist evaluation with regular follow up as you may need different approach,check aaaai.org for an allergist