#3 It makes you seem “easy.” All relationships have milestones, from the first date to the first kiss to the first time you see each other naked. Having sex at the first instance eliminates that particular milestone since it’ll be all over in the first 12 or so hours.
Also, some people treat sex like a conquest, a challenge to overcome. If you yield to your sexual desire too easily, then your date may either think that you’re an easy lay that doesn’t even need the promise of a next date or that you’re just looking for a no strings attached relationship.