That is confusing to me - to allow openly gay believers to join a congregation, but at the same time refuse to ordain or marry them? I mean, I know the risks for the denomination to take the next leap, so I get it...
I cannot speak to "risks for the denomination," but I might give a rationale for the decision which we have made as a denomination.
I believe that the majority of the denomination is of the opinion that the practice of homosexuality is a sinful behavior.
It is one among many sins. We want our members to not be sinners and we don't want them to endorse sins, but we recognize that all are sinners. So, we don't exclude sinners from being a part of the church. If we did, there would be no members in the church.
Likewise, we recognize that the same is true of pastors. But, we do expect that pastors even if not perfect are going on to perfection. This is actually one of the questions put to each and every person who is ordained in the United Methodist Church. Another of the questions put to us is if the candidate has read and can commend our rules.
Now, if a pastor was to be ordained who engaged in something that the church saw to be a sinful behavior, then if he/she was going on to perfection that would mean be willing to confess that sin, repent of it, and leave it behind. Now, we can ask that of people with regard to lots of behaviors, but if the person does not even acknowledge it as sinful behavior, then there would be no reason to change the behavior. Most who practice homosexuality feel that the behavior is NOT sinful. Thus, they could not in good faith promise that they were going to commend our rules, nor could they promise that they were going on to perfection and refrain from practicing that which the church has identified as sinful behavior. They might claim that they were going on to perfection and not see a need to change in this area of their lives, but that would leave them a leader of the church who did not practice what the church taught. So, to me at least, it seems more honest to say to pastor that if they cannot (or perhaps more honestly, will not) keep our rule, than they are welcome to come in and be a part of the congregation with the rest of us sinners, but it is better if they not take on the role of leader in the church.
Similarly, if a person was addicted to any other sin so that they were to refuse to acknowledge it as such and not be willing to at least try to change his/her behavior with regard to it, as a member of the District Committee on Ordained Ministry who reviews candidate, I could not recommend them to be ordained. We do not demand that all candidates be perfect, but we are serious about understanding ourselves as called to be going on to perfection. They are not just words.