Not only do you need to be married, you need to be married for at least 1-2 years and you will need an attorney.
actually, that's not correct for TX.
in TX, you can terminate dad's rights without a step parent adoption. certain standards have to be met, but it can happen.
but from i've read....you do need to be married in order for a step adoption to take place. can't adop as a step parent if a marriage never occured in the first place.
BUT the fiance cannot adopt the child unless he adopts as the child ONLY parent while mom AND dad terminate their rights. part of a process.
so OP, you're best bet is either terminate dad's rights....OR do a stepparent adoption while terminating dad's rights after you are married for a year or so. might be cheaper that route.
however i still suggest getting an attorney with any TPR and/or adoption.