Welcome to the Forum. I'll be happy to comment. Your approach is thoughtful and, as you point out, there are ways to modify the risk for acquisition of STIs, as well as to moderate their consequences should you get infected. The major issue in play here is the matter of just how "open" are the relationships of your other partners. Risk for STIs is a numbers game and as persons have more partners, who in turn have had other partners, the risk for infection goes up. Testing for STIs before entering into such relationships is a good way to reduce risk as is periodic checkups’ Subsequently as persons have sex with other partners who may have had sex with others themselves. It appears that the decision/preferences of your potential partners do not include condoms which are the best way to reduce risk for infection and conversely, choosing not to increases risk.
As for your specific questions:
1) Within our sex-positive community, condoms for fellatio are rarely used. Is this risky behavior? It seems as though STIs other than Herpes that could be transmitted can also be treated. Therefore, is no condoms for oral, along with regular testing, a safe approach?
The risks for STI transmission through oral sex are lower than for vaginal sex, both because of the fact that such infections are somewhat less common than genital infections and because the act itself is less likely to lead to transmission. Herpes is a consideration for persons who have not yet had HSV-1 infection (about 60% of people have, even though most who have been infected do not know it). Gonorrhea occurs infrequently and oral chlamydia is rare but does occur. In both cases these infections can be asymptomatic and therefore testing is important for detection, treatment and prevention of transmission to others. Syphilis is very, very rare and transmitted by direct lesion/sore contact. Finally, there has been much in the news recently about oral HPV infections and cancer risk- the risk of this occurring is quite low- here is a like to another post in which Dr. Handsfield discusses this topic -
http://www.medhelp.org/posts/STDs/HPV-and-oral-sex/show/1515473 and
http://www.medhelp.org/posts/STDs/Oral-HPV-Cancer-Risk/show/1512873 .
2) Does it make any difference one “spits or swallows?”
No, spitting vs swallowing oral sex is not known to impact STI risk in any way.
3) Surprisingly, one partner of ours who does engage in sex with many others is not on birth control. (She is 41.) Because of condom usage failure rates (15% according to Wikipedia), my wife and I had previously agreed that we would only have intercourse with others who were on some other form of birth control, in addition to condoms. However, would another reasonable approach be to combine condoms with the withdrawal method, figuring the yearly failure rate to be about 2.7%? (.15 condoms * .18 withdrawal = .027 combined?)
Withdrawal is ineffective as a pregnancy prevention measure and wold not add to reduced risk. The only other thing I would add to your thoughts above is that as persons become older fertility (i.e. the likelihood of becoming pregnant) diminishes. Thus this woman's risk for pregnancy may be lower than that of a woman half her age.
I hope these comments are helpful to you. As you imply, being attuned to the possibility of STIs and regular testing will be an important of assuring and preserving sexual health for you and your wife. EWH