Perhaps not harms but there are several things to consider if you are not careful enough. It's all relative, it all depends on your frame, on what you really want, on how much you care about others, on how skillful you are...
* STD's. If you sleep around with many girls and are not careful, you may get STD. Chlamydia? Ok, no big deal. How about herpes? That may be quite unpleasant. What about Hepatitis? HIV? Compare to conservative guy who doesn't sleep with many girls. Some girls may want you to show proof that you don't have any STD's first before she sleeps with you. Those are usually girls who care about who they sleep with, meaning quality girls. Imagine, you get to date this hot girl, everything is great, yet she rejects you just because you have herpes...
* Sexual harassment, especially if you make good $$$ and have high social status. You will become sexy and confident, you will be going after women and people will notice. However, some women may want to use it to their benefit. Women are smart, women are not stupid. They can set you up, you have no idea, they are way smarter than you. She will make it seem that she is into you, she will make it seem that she wants you, but then the reality hits. She will deny everything, she will create different story that what she made you believe... She will lead you on, she will let you touch her and kiss her, and then she will make a stalker and pervert out of you... The silly/cute/sexy girl you knew will turn on you like a cold beast with no mercy. I sh*t you not. Look at classical examples like Clinton or Trump. All these 'poor' and 'innocent' women that everybody feels sorry for... Right... Sexual harassment laws and rape culture is very real today as everybody feels sorry for those poor innocent girls... You may believe that you are this cool and great sexy guy who every girl loves and chases, but beware of what you wish for...
* Some good quality girls that you could otherwise date (and possibly marry, if that's what you want) will avoid you. Good quality girls who are more conservative and perhaps religious usually don't want to be associated with a guy who sleeps around. Don't make me wrong, they may be very attracted to you, they may want to sleep with you, but at the same time they will reject you because you sleep around and she simply can't trust you as a long term potential partner. The girl doesn't see only relationship between you and her, she also considers all her family and friends, and if you don't fit because you are too cool and everybody knows that you like girls, well, you just don't fit... Think about it, that is huge, because many guys want conservative girl who doesn't sleep around, and these girls are not so easy to find these days... You could be missing a lot...
* For the same reason, you may be perceived as attractive guy by many girls, yet they won't want to date you because of slut shaming. Which girl wants to be seen with guy who slept with lots of other girls? Only sluts don't care. Many girls in that particular envirnoment may flirt with you, talk sex left and right, but none of them will want to go out with you because everybody knows what you are doing... Girls talk, the word spreads very fast...
* Your personality will change. You will have different mind set. You may lose some good friends you knew for years, they will not want to be around a guy who is now much more successful... At this time you are so to speak 'calibrated' to society, you are part of the main stream. Once you develop your own mind and style, you may become even quite awkward. But again, that's based more on personality - some guys may not be affected at all, others just become somehow unusual (if lucky). Yet another ones can be perceived as very weird... Let me tell you, once I noticed a guy who tried some NLP trick on me to develop good rapport, I recognized it right away because I was familiar with it... That is a CREEPY feeling man, you know right away that there is just something very wrong with that person...
* For the same reason (and depending on your personality), you may become lonely. There is lots of seduction enthusiastic here online, but it may not be so in area where you live... Most people may not understand where are you coming from...
* You may break some girl's heart. Girls are just humans, if you have a good heart yourself you don't want to really hurt other humans. When you develop a good connection with the girl and then walk away from her, she will hurt... You may get to known girls who are not as sexy or good looking (say 4-5/10, some extra pounds), but they will perceive you as very sexy and cute. You may relate quite well to these girls on emotional level, there could be really great vibes and lots of fun. They too want to sleep with you, but, if you reject their attempts they may feel miserable about themselves... Sometimes it may be tempting because you feel the power as they are after you, but don't flirt with these girls, don't lead them on. They will hurt, you won't...
* You can even mess up some girl's life. She may be taken, married, and by getting to know you she may break up with the other guy. Perhaps not even because of you directly that much, but because of the feeling that she was cheating with you on the other person who she thought she loved, and now when you are around or gone, she has lots of regrets... You can also quite fuck up her mind, because a girl who is in relationship and starts thinking about another guy (you), may simply not feel good about herself... Simple thing to fix this is to avoid any girl who is in LTR from the very beginning...
* If you are an asshole, you are an asshole. Some girls dig it, they will love you for it, but most people won't like you. Not too many people want to be around assholes and pricks, no matter how cool they are. Again, they may respect you, they may perceive you as a high value man - but you will be lonely, unless you find another friend(s) with similar qualities...
* Rejections... Some fucking hurt, and I'm not talking about girl not giving you her phone number. I am talking about girl who you get to known well and may consider as good long term partner, but because of your unusual behavior (too smooth, knowing too much, have too many skills,...) she will put her breaks on. That's also a pain, not because you don't know much, but because you know too much, because you over-game her, everything you do is just right, you are too smooth, you are not making any mistakes she is used to... She will feel that she has no power in the relationship, thus she will exclude herself from it...