You can't look to the future until you let go of the past.  It may seem like harmless "flings" with your ex, but if the relationship isn't moving toward reconciliation, then it's binding you to failed love.

This holds you back from finding the right person to satisfy your needs romantically, sexually and emotionally.  Moreover, it prevents you from wholly acknowledging the end of your marriage with clear focus towards a better life and personal happiness.

It's just sex.

Sex is an important component in a romantic relationship.  But don't sell yourself short by referring to it as "just sex".  Most married couples shared a great deal more than sex for many years and those feelings don't diminish with continued intimate relations.  

It may seem like you are safer having sex with your ex rather than someone new.   However, if you're looking for a purely sexual relationship the choice to do so with someone you formerly loved is a bad decision.  It diminishes the love you once had to "just sex".  To fully respect each other and the life and love you both deserve, seek more than break-up sex with someone with whom you no longer love.

I need time to move on.

The time you need to move forward doesn't start until you let go.  These two aspects of the divorce cycle are part of the last two Stages of Divorce.  First, you must recognize what went wrong and have the opportunity to process your feelings in order to find lasting love.  As long as you and your former spouse rely upon each other to fulfill natural needs for bonding, intimacy and companionship, the longer it will take to deal with the loss and establish a plan to truly move on.

We both enjoy each other's company.  What's the harm?

A close relationship with your ex is one thing; continuing to use each other for the typical benefits of a couple is another.  Most times, the benefits of the marriage weren't the reasons it failed.  

Remind yourself why you decided to divorce.  If both aren't committed to working through those problems, then you're stuck in a relationship that will always be missing some of the most important components of a loving relationship. 

Are you happy with all that the relationship has become?  Usually, in these cases, the answer is no.  Even if it is all you currently seek, a former lover is rarely a good choice.

We both date other people sometimes, but still have a close relationship.

Again, being close is great!  Intimacy with your ex while engaging in a relationship with others is unfair to everyone.  It doesn't clear away feelings from your past and allow for your new relationships to develop.  It may seem like your intimate relationship with your ex hasn't influenced the failure of your new relationships, but it does. The longer you and your ex are apart, the more you may seek a new lasting love with someone that wants more than what your ex can give.  But, until you fully commit to "moving forward", your occasional flings will always affect your new relationships (even if your encounters don't occur while you are dating someone else).

I'm willing to work out our problems and I think my ex may want the same.

Sometimes it's just easier to stay with someone that's familiar and comfortable.  Don't expect a rekindled marriage to be easier than starting over because it's familiar.  It takes a lot of work.  Make sure your mutual interest in "working it out" is clear and both are equally interested in the same outcome.

Communication is a huge problem in many relationships that have ended in divorce.   While your expectations of working it out may be one thing, your former spouse may have a completely different idea.  Discuss your expectations, issues that led to your divorce and ideas for working it out.  It may be a good idea to seek some professional guidance with a therapist who has experience in situations like yours.   This may help determine if your spouse is as committed as you so that you both can implement a plan for an improved lasting relationship.