You are right that there is double meaning in the line
For having lost but once your prime,
The double meaning works because “once” means not only “at some point or period in the past” but also “at one time only” (OED). In a discussion of virginity, a reference to losing something “but once” has a clear implication, and “prime” has the meaning “the beginning or first age of something” (OED) which we can take to be a metaphor for virginity.
However, we have to be careful in teasing out the consequences of the double meaning, because “tarry” does not mean “relax” but rather “wait; linger; abide”. And the idea that virginity might be something a young woman might be anxious about, so that she would relax once it is lost, is a modern idea, and it would be anachronistic to read it back into Herrick’s time, when female virginity was conventionally conceived as something precious, to be preserved for marriage.
So I think it is better to take the last two lines in the context of the stanza, which is explicit that it is advising the virgins to marry:
Then be not coy, but use your time,
And while ye may, go marry;
For having lost but once your prime,
You may forever tarry.
So the two meanings are: “once you have lost your youth, you may wait in vain to marry (because you are no longer beautiful)” and “once you have lost your virginity (outside of wedlock), you may wait in vain to marry (because you have lost your respectability)”.
These conventional morals seem rather disappointing after the vivid imagery of the first three stanzas (“gathering rosebuds” as a metaphor for sex, “the glorious lamp of heaven” as a metaphor for youthful vigour), and it is natural to look for something more out of the last stanza.
The metaphors in the poem are all about the passage of time leading to decay and death: the flower will be dying tomorrow, the sun races towards its setting, time turns the best to the worst. With these progressions in mind, wouldn’t it be better to tarry? So rather than interpreting the last line as a warning (“you may wait in vain to marry”) we can find in it an opportunity: “through sex, you may be able to delay the progression of time” (though in this reading “forever” would be hyperbole: perhaps if the sex were good enough it would seem like forever).
The question asks whether there is a “canonical” reading of the poem. I recommend not worrying too much about this: poetry is an art form which exploits ambiguity of language to create and support multiple readings. So long as an interpretation is grounded in the text and convincingly argued then there is no need to be concerned whether other interpretations exist. The fact that a poem has a second (or third, etc.) reading does not mean that the first reading is wrong.