At the moment, there are no PC long-term relationships. That is future content. The BFF to SO content is long & somewhat convoluted.
1)Whenever possible take the 'talk about sex' option when hanging out with them. You can hang out any night but Friday, from six to eleven.
2)Find out the 'Friday night Party Secret'. Mostly this is just a matter of going to the party regularly & spending time there. Once you know it, you have to share it with your BFF & deal with the fall out. This takes two consecutive parties to complete.
3)Keep taking the 'talk about sex' options as they are available, it feels as though they are somewhat less frequent with the new patch, but that could just be that there is so much MORE to do now... Eventually, you will reach a 'tipping point' moment, choose wisely.