If I get a semi erection from a text message, does that warrant a full relapse?

If your question is whether or not a text message can be used as a justification to relapse, then I would say there's no good reason or justification whatsoever to relapse.

If your question is whether your semi erection from a non-sexual text message is considered a relapse, then I would say you need to figure out your parameters. Based upon your counter, you have three questions to ask yourself:

1. Did you look at porn?
2. Did you masturbate?
3. Did you orgasm?

If the answer to those 3 questions are no, then there you go.

On a side note, getting an erection is a natural function of our bodies. I've not ever heard of the parameter: No Erections. Not only is that unrealistic, but I fail to see how it's of any value. A normal physiological response is not a cause of addiction.