In his first comment on the ban since he admitted his gay past, the former secretary of state for defence said it was in "the national interest" that it remained in place.
Gay rights activists accused Mr Portillo of hypocrisy after he told BBC Radio 5 Live that he had upheld the policy on the advice of the military chiefs of staff.
His remarks were just one of several hiccups on the first day of full campaigning for the seat, which was left vacant by the death of Alan Clark.
The day after the triumphant night before had started well, with Mr Portillo up bright and early to give doorstep interviews to the waiting media.
Clearly aware that his return to Westminster would be far from smooth, he warned that Labour would conduct "an intensely personal and unpleasant" campaign against him.
After a brief meeting at the Kensington and Chelsea Conservative Association office, he went on the first of several walkabouts around the constituency.
With a performance as slick as his soft quiff haircut, he efficiently glad-handed chiffon-scarved ladies and Barbour-jacketed young mothers on the fashionable King's Road.
Even a trip to meet the impeccably dressed Chelsea Pensioners at the nearby Royal Hospital was trouble-free, with the Tory candidate having the good sense not to refer to a certain SAS speech.
A series of tricky television and radio interviews, including the 5 Live appearance, were then negotiated at Millbank, where he dodged questions about the Conservative leadership and stuck to the party line on Europe.
However, Mr Portillo's 200-watt smile was sorely tested later when he took a stroll through Holland Park, home to squirrels, peacocks and - for one day only - Peter Tatchell, leader of gay rights pressure group OutRage!
Mr Tatchell, who has vowed to fight the seat to highlight his opponent's "homophobia", attempted to engage him in light conversation among the autumn leaves and conkers.
Tory minders were clearly uninterested in the finer points of the age of consent debate and pounced on Mr Tatchell, gripping him in a tight headlock while Mr Portillo ambled silently away.
As the Tory party moved the writ for the by-election in Parliament, and the other parties unveiled their own campaigns, the former Enfield Southgate MP's rocky road became even more evident.
Labour vowed to use its secret weapon, Stephen Twigg, the man who defeated Mr Portillo in 1997, to help Labour's candidate, Robert Atkinson, in his attempt to overturn the 9,000 Tory majority for the seat.
As well as Mr Tatchell, the Pro-Euro Conservative Party leader, John Stevens, has already pledged to contest the poll on 25 November. Other "Stop Portillo" opponents are likely to include the UK Independence Party and the right-winger Adrian Rogers, who is expected to run on a Conservative Family ticket.
The big event of the day was next, a photo opportunity with William Hague at Conservative Central Office, at which reporters were barred from asking questions.
Unlike the last time the two men shared a major media event together, Mr Hague was not wearing a sodden anorak and Mr Portillo tried his best not to look like a smug prince over the water. But as they posed with fixed grins in Mr Hague's minimalist office, the picture of determined unity was undermined by the Tory leader's refusal to share a sofa with his undeclared rival.
One could get several million cigarette papers between the pair as they perched on couches at 90 degrees to the each other. "We're quite all right as we are," Mr Hague told photographers desperate to snap them side by side.
In his dignified selection speech the night before, Mr Portillo had promised to fight on behalf not only of the wealthy but also of those "who have great problems". What he didn't say was whether that included helping Mr Hague with his own great problems.Reuse content