Colorado courts don't care why you want a divorce. This is a "pure" no-fault state – it doesn't recognize any form of marital misconduct as grounds to dissolve a marriage. If your spouse married you then barricaded herself in the spare bedroom and refused to engage in sexual relations with you, you might have grounds for annulment in the state, but you can't cite it as grounds for divorce.
Irretrievable Breakdown of the Marriage
Colorado's sole ground for divorce is that your marriage is irretrievably broken. If your spouse disagrees, this won't stop the divorce; you don't need her agreement or consent. If you've lived in the state for 90 days, you can be divorced 90 days after your spouse is served with a copy of your divorce papers, assuming you can resolve all your marital issues. The court doesn't care why your marriage broke down, including a lack of sex.
The Option of Annulment
Because Colorado's no-fault provisions are so uncomplicated, it's usually easiest to just file for divorce if you want to end your marriage. However, lack of sex can be grounds for annulment, or voiding the marriage, under some circumstances. For example, if your spouse is incapable of consummating the marriage because of some physical infirmity, and you didn't realize this until after marriage, it can be grounds for an annulment. If your spouse never intended to have sexual relations with you, and you found this out after the wedding, this can be fraud, which is also grounds for annulment. The burden of proof for an annulment is significantly greater than for a divorce, however. You must detail and establish your sexual problems to the satisfaction of the court.