Is it OK not to have CTO co-founder?

If your product or service considers technology as a competitive advantage compared to other marketplaces , yes it is better to have a CTO who can leverage technology to build capabilities. E.g if you are buyer-seller matching is based on machine learning backed algorithms, you need to retain the tech guy. Does not mean he/she has to be a CTO. You could use equity power too.

If you business is just another market place and the your execution power is the differentiation, then you could live with a agency/freelance company

My co-founder and I are not programmers. I have a design/marketing background and he has a business/sales background.

We hired a skilled developer to work on the product and after he proved himself and we were approaching PM-fit we promoted him to CTO and gave vested stock options.

Some candidates may want the title right away, or stock options right away, but in any case you don't *need* a CTO co-founder as long as you have a very good lead developer hired.

Any skilled, experienced programmer who believes in what they're building will treat the product like their baby, give them the title and mentorship to grow along with the company and you can do well without a technical co-founder.

Based on what I have been observing in a number of cases, generally speaking, it really depends on what the startup is doing or proposing to do.

When the required technologies/competences can be easily found in the market, it is highly possible to survive without CTOs and/or co-founders without specific experience/competence. When the new Technologies are being developed as part of the activity of the startup than it is highly recommended to involve people with specific experience/competence (and vision, enthusiasm, capacity to integrate new solutions, etc...), since the earliest stages of development.
In both cases it is highly necessary to invest time and attention in the careful definition of requirements and specifications for the work that (external) development teams will be asked to implement.... this means that if no CTO at least someone with a clear functional view must be present (internally).

In your specific case, I suppose that to be really original creating a "marketplace for freelance developers" you really need to involve asap very skilled and very creative developers who, at same time, have the capacity to specifiy and describe their "dreaming marketplace". whether they need to be co-founders or not is largely a matter of "relationships" and "sharing of risk"...

If the answer to the following 3 questions is "yes", you should get a CTO. If answered "no" to at least 2 of them, you could probably manage with a freelancer or part time employee.
1. Is there a lot of (technological) work needed (the "Work")?
2. Is the Work ongoing (for at least 1 year)?
3. Do you need someone with a unique talent to do the Work (versus the fact that many programmers can do such work)?
Despite the above, if you're looking to raise capital (with Venture Capital firms), you should consider getting a CTO co-founder or full time employee as VC's are reluctant to fund startups/ventures that don't have a CTO as part of the team.
If you do decide to take on a CTO, make sure that you use a reverse-vesting mechanism (there's information on this online but I'm happy to help).
I've successfully helped over 300 entrepreneurs and I'd be happy to help you as well. Good luck