Herrick uses "rosebuds" rather than roses since rosebuds are not fully bloomed yet...much like the virgins to whom the poem is addressed.
The speaker is probably an older male who is advising the young women of the community to hurry off and marry while they are still young and beautiful. To "tarry" too long will mean that time will ravage them and cause them to age to the point that they will not be able to attract a worthy husband. The tone of the poem suggests his age, as well as his opinion of the effects of age on the young women.
The argument is simply that time is not kind to women, and that their only chance at marriage is to do so quickly when they are still lovely enough to attract a husband. Frankly, I do not see the support in this argument nor do I find it convincing. In the first stanza, he tells them that rosebuds are pretty today, but dead tomorrow--parallelling the virgins' beauty. In the second stanza, the speaker talks of the sun's race...signifying the passing of time and the loss of youth and beauty. In the third stanza, the speaker says that youth is better (blood is warmer) so don't waste time playing hard-to-get. He carries this over to the fourth stanza by saying "use your time while you can and go marry!" His argument is that if you wait too long, you will always be unmarried...a spinster.