In my candid opinion, fating more than one partner is a toxic lifestyle. Aside from the fact that you might be prone to different forms of STIs or STDs, coping with lying and denial just to maintain both relationships can be extremely stressful and tiring even if you might see yourself as a Casanova, the mind stress would definitely get to you.

Having more than one partner is not advisable, it opens up the possibility of having different form of stress. Physical stress, mental stress and psychological stress. Physical stress occurs from the fact that you'd want to please both partners in anyway you can just not to give up the truth that you are a double timing Casanova. Sexually satisfying two partners requires a lot of strength which comes at a price, you get worn out always.

Mental stress on the other hand stems from the fact that lying, maintaining the lie and selling the lie to your partners for them to believe you requires a lot of thinking and strategizing which can lead you into a dull mental state, aggressive or even moody.

Furthermore, psychologically, one cannot cope with having to maintain two relationships at the same time. the stress on the mental state can cause a psychological breakdown of such individual.

Apart from the stress mentioned above, dating more than one partner opens up the individual to the possibility of contacting STIs or STDs which happens to be a product of having multiple sexual partners.

There are other negative consequences like physical fatigue, high blood pressure due to excessive stress, falling sick often etc.

In a nutshell, it is not advisable to have more than one partner because it's just like sitting on a keg of gunpowder with an alresdy lit matches at hand.