This guide presumes three things,
A: You are doing the MOOsex with other people.
Two: That you do not know how to do this constructively, and with a plot either internally or externally.
Peaches: You may not have had a relationship IRL with which to base shit off of.
So here's my two step method for determining if you are capable of doing this RP constructively to yourself OOCly.
First off, if you yourself are incapable of preventing bleed or at least separating and recognizing bleed, and ascribe intimacy and traits in character to the out of character player. Do not engage in an RP'd relationship. It gets weird for one party, and depressing for the other at the point that feelings get involved OOCly. Basically, if you're not capable of playing the feelings without having the feelings, then you're probably fucked in regards to RPing anything longer term as far as relationships.
This does not mean you, OOCly, can't like the people you are talking too, and MOOsexing with. It just means separate the character from the player and vice versa. Your characters are getting intimate, you are just a scrub behind a keyboard who doesn't know anything about the other scrub behind the keyboard other than assuming that possibly they are also fondling their genitals in their seat while doing this.
Second off, are you capable of communication and acceptance? Because if you are not, then that's another strike for this. People often use online relationships to work through their own issues, kinks, or etc. And not understanding that, and communicating your own desires, should exclude you from doing something like this.
Onto the meat... Here's my three simple tips for making sure this stuff works out in a fun and non-boring for everyone way.
Stay distant ooc, and have a goal IC.
From the start of a relationship decide that you have a goal for your partner in it ICly. Don't expressly communicate that goal, but at least indicate that you are developing your character and there's with plans and ploys. Stay distant OOCly from these plans, because if you are trying to do this OOCly to a person you're a cunt. And also abusive.
Goals are basically flex tape for patching bad RP or sex focused out. Think about what you want to do with their character, and your character and work towards it with RP? Do you want them to be a serial killer? Do you want to reform the Mix Monster? Are you trying to brighten the life of someone? Do you want their character to be dependent on you? Are you manipulating them for a job? Are you infusing peer pressure into the relationship, to generate an addict for your product?
Have a goal. And set it up and work towards it. But be ready for that to fail.
Setup safe words both OOCly and ICly, be ready to fade to black because they aren't feeling RP today. Be conscious of their schedules work with them to help them Communicate communicate communicate, if you're the type that sleep in the same room as their character, say Night in local ooc as they go to bed or log off oocly. Remember you're a consistent part of this persons ICly life and OOC life, so keep the you and your character separate. Do your best to make both of you comfortable. Because at the end of the day if one of you is uncomfortable with this type RP you are doing it wrong.
Don't just MooSEX, integrate the relationship into your character's life, be a dramatic fuck, someone fucks with the girl you're screwing into a serial killer murder pet ICly, make sure that you, if you're more capable in combat are on their side. Visibly to them at the very least. Fuck with the people who fuck with them. Put out the fires they start diplomatically. Integrate yourself in such away that it's an actual relationship or partnership for your character, if it is something like that for a long term.
I think two pages is enough for this for now, I may come in and add a bit more in another post but I don't want to slam you guys with like a full on five page guide here, even if I could probably easily do that on this subject.