Single
Pros:
Complete freedom, you are beholden to no one. You set your own agenda, plans for the evenings, can follow your career to different cities
Women: Everyone and anyone as long as you have game/cash. You don't need to think 'oh if I wasn't in a relationship, I would hit that'. You see it, you do it (So to speak)
Kids: Kids can be a joy beyond compare. They can also be an anchor tying you down to one place, one job, one lifestyle your entire life. They are expensive, needy, and non-refundable. If they turn out to be assholes or deadbeats, it's partly your fault. You may have to pay for their mistakes your whole life. Sometimes its replacing a lost ipod or cell phone, sometimes it's bail money/rent/rehab
Marriage: By staying single, you avoid marriage. Again like kids, marriage can be a blessed thing. It can also suck the soul from your body, and leave you broke and alone. Even the most cordial divorces turn ugly, and can cost you in many ways. No marriage, no commitment. You can walk away anytime, whether it's through the hotel door after a SP appointment, or from a civilian relationship. If you manage your money, you are completly fiscally independent and can choose (barring unexpected events) how to spend it. If you get divorced, she's entitled to half.
Cons
No way of passing on your family name. If you want kids, but don't want marriage, it's tricky. Adoption is very difficult for single dudes.
Kids can be amazing. They're something you and your partner created out of nothing, and they become a reflection of you, and an outlet for your hopes and dreams. Not to mention they can support you in your old age
Having someone to share your life with. Sleeping alone can suck. Beyond the obvious, if you die alone in your apartment, it will take a bad smell for people to realize it, it can be amazing to share your life with someone. If it's a good relationship, you can share anything without judgement, experience all of life's wonders and joys with a partner that you've chosen, and that has chosen you. Being in a relationship can make you feel like you're part of something bigger than you
Friends: As time goes on, and you get older, gradually most of your friends who were supposed to be 'single until they die' get married, have kids. They have a new life which you're not part of anymore. Aside from the pity and regret they'll have for you, they aren't capable of dropping everything to grab a beer after work, or catch the Leafs game. They have less time for you, and even when you do hang out with them, it's short
Financial: Sometimes it's nice to have another income in the household. Better apartment, better neighborhood, better car